This occurs after Chapter 6, where Shirou decides to stick with Saber instead of going to Rin for help...

Shirou thinks for a decent amount of time on what he should do next, and while it was likely that Rin could figure out this shrinking business, it might be a better idea to trust his servant. So he looks up at Rin and says “Even though we came all the way here, I’m having second thoughts about burdening you with our problems. We can deal with it ourselves. You just focus on winning the war, since you’ve been preparing your whole life for it…”

Rin frowns when she hears that, before saying “Well that’s disappointing… And here I thought I’d get to keep the bug-sized Shirou to myself. But since you’re in such a sorry state, I won’t seek you out as an opponent if you don’t. Deal?”

Shirou nods and says “That’d be a huge relief. Thanks Rin!”

After that, Saber respectfully bows and puts Shirou back in her boot, before quickly returning to the Emiya household. This time she made sure that he was closer to the toe area, so he had a much smoother experience, though he still wondered why he liked being inside her boot so much. After a short while, mainly due to how quickly Saber was moving, they make it back to the house.

Back at the Emiya household...

Then, Saber removes him from her boot and places him in her palm. He expected her to be upset at him for being stubborn, but she actually looked pleased at the current development. He then sighs and says "Sorry about that, after you took the time to take me to Rin’s, but I just felt like we should deal with this together… Is that okay?"

Saber responds by warmly smiling and affectionately rubbing him against her cheek, while saying "Oh Shirou… It makes me so happy that you have this much faith in me. I will not fail you!"

Shirou laughs as he is rolled against her smooth and warm cheek, while saying "I get it Saber!" But then she suddenly presses him harder against her skin, as if trying to hug his miniature form, also unintentionally silencing the tiny master. After that, he says "You can stop Saber! I understand!"

When Saber hears him, she immediately releases him and looks down in embarrassment, before saying My apologies Shirou… I allowed my emotions to get the better of me."

Shirou just smiles in response and says "It wasn't that I disliked what you did; I'd just appreciate a warning first… But what should we do first? The Holy Grail War is still going on."

Saber nods and says "I suggest that while the conflict is in its early stages, we go out and purchase food supplies."

Shirou grumbles and folds his arms, before replying by saying "You're just saying that because you like to eat…"

But when Saber hears that, her eyes widen as she becomes overtaken by passion, before yelling "Of course!!" The force of her breath caused Shirou to be launched off her palm and toward the floor. When Saber saw this she panicked, and instead of catching him since she was quick enough to do so, she instead calculates his landing zone and kicks her brown hiking boot right there.

Shirou lands inside her boot with a thud, where he is immediately hit by a potent odor that hung in the air, likely because she just took off her boot. The ground was also highly sticky and damp, but also incredibly soft. And when he looks up, he sees Saber staring down at him with a heavily guilty expression. She then rubs her hands together nervously while saying "Are you okay, Shirou?? I thought this was the best course of action to diminish the chances of injury…"

She then reaches into her boot and lifts him out of it, causing Shirou to dust himself off and say "You didn't mean it, Saber. And I'd rather land in your soft boot than the hard floor any day…" But then he sighs to himself and adds, saying "I don’t know if I’ll ever adjust to the smell though…"

When Saber hears that, she pouts and says "It's not my fault! As a warrior I need to remain active at all times, and those boots support the rest of my body perfectly…" Then she frowns and adds, saying "But I'm sorry I caused you discomfort, Shirou…"

Shirou then shakes his head and says "No… it wasn't that I disliked it. Or at least I don't think I did… or…" Then he loudly grumbled to himself, causing Saber to stare at him curiously, before he says "Why don't we save that for later…"

Saber nods and says "Understood!" Then she pauses for a moment, before saying "As I was trying to say before, gathering food supplies is our top priority. I am a knight who needs to be ready for combat at any time, all to keep you protected. I will need plenty of food, and it will become more difficult to obtain it as the war drags on."

Since Saber had a good point, Shirou nods and says "You've convinced me… Take my card to the nearby market. I'll tell you what we'll need to get…"

Saber happily grins in response before saying "Excellent decision master! Now I only need to decide on where to keep you…" She starts looking around, trying to determine the best hiding spot for him, before smiling and saying "I've come to a decision…"

Shirou nods and says "What's your idea, Saber?"

She starts to unbutton her blouse shirt, slowly revealing her decently sized chest, while saying "If I put you in my bra, you will be perfectly hidden. No man would be foolish enough to, as this age's slang puts it, 'check me out' while I'm shopping."

When Shirou hears that idea, he immediately backs away and says "We can't do that!!" But then he thinks for a moment and asks "How do you know slang?"

Saber sighs and says "The Holy Grail fills us with knowledge of the current age, and I presume that slang was included for whatever reason..." And then she adds, saying "And why can we not do that?"

Shirou folds his arms while blushing, before saying "Because it'd be inappropriate!!"

Saber shakes her head and says "I disagree. It would be a secure and hidden place to keep you." Then she smiles and adds, saying "Besides, even as a perfectly average human, you attempt to fight for your ideals and protect the weak." She then begins to gently rub Shirou's head with her finger, before adding "I can think of no other man in this age to keep that close to me… Please allow me to keep you close to me…"

When Shirou hears that, his eyes widen as he says "Saber, what do you mean…?"

Saber, understanding that her master was kindhearted but incredibly dense, just smiles and says "What I mean is that I have the utmost respect for you as my master. Please trust me to keep you safe and happy…" That was enough for her, for now at least.

After that, Shirou sighs and says "I understand Saber… I'll continue to trust your judgement." But then his eyes widen as he adds, saying "But how will I communicate with you??"

In response to that, she smiles and says "Now that I know how small you are and will know where you are, I should not have any more trouble with contacting you telepathically." Then, she psychically speaks to him, saying "See…? You can hear me just fine… Try to send one back…"

Shirou nods, and then telepathically says "Is it working...?" She nods and gently rubs his head, before continuing to undo her blouse shirt. After that, she fully unbuttons it revealing a plain blue bra, causing Shirou to blush. Then she pulls one of the cups out while lifting him over the gap she just created, giving him a clear view into the deep canyon down below.

Then she smiles and says "I will see you as soon as I get back. Be safe, master…"

Following that, she drops Shirou into her bra cup, feeling uncharacteristically excited for the whole ordeal…

When Shirou lands in the bra, he notices that a there was a unique aroma that hung around, which seemed to have a calming radiance. But when he looked in front of him, he saw Saber’s breasts in all their splendor, and they were breathtaking to the tiny mage. He wasn’t the most knowledgeable about the general subject, but he guessed that she had B-cup breasts normally, though at his current size they looked more like mountains. And her skin looked perfectly smooth, as if she were a goddess of beauty, though he wondered if that was because she was summoned as a heroic spirit. Whatever the cause, it made him want to compliment her, since he was fully aware that she doubted her own femininity.

Saber was watching him stare at her motionlessly, causing her to ask “Are you okay, Shirou?”

He nods, before saying “Sorry for staring, Saber… But you really are beautiful.”

When Saber hears that, her eyes widen, before she quickly coughs and says “Oh… that was incredibly kind of you, Shirou…” And once she finished saying that, she smiles warmly and says “And I don’t mind… But I need to get going now, so get ready, master..”

After that, Saber releases her bra, allowing it to attach itself to her breast. That caused Shirou to be pinned against her boob, which was both soft and warm, making him feel like he was enveloped by a fleshy pillow. And as Saber re-buttoned up her blouse, he found himself pressed deeper against her chest, to the point where he couldn’t move a muscle. It didn’t bother him though, partially because he got to be practically smothered by Saber’s breasts, since he always did find her to be incredibly beautiful. But it was also highly comfortable in this position, though the growing heat was starting to be distracting, making him hope that she wouldn’t sweat too much while she was out.

Before long, Saber had fixed her blouse shirt and put back on her brown boots, so she left the Emiya household toward a nearby store, hoping to find enough food to last them through the rest of the war...