Okay, this is an idea I had (okay, technically it's a slight modification on a friend's idea) where, basically, as punishment for having utterly crushed a girl's heart, our protagonist gets shrunk to near-microscopic size by a friend or family member of hers. The only way to get back to normal is if the protagonist is completely forgiven and gotten over- which won't be easy, especially considering that a manipulative ex has reentered the picture and is using the heartbreak as an excuse to get laid!

Being me, I hope that at least one of her two love interests (tiny or ex) is another woman (favorite trope for reasons), and I really like theme naming. Beyond that, it's almost entirely up to you. 

While I kinda expect at least the first chapter to have our heartbroken girl unaware of our protagonist's existence, it's writer's discretion as to how long this lasts. The concept as I pictured it was where the original shrinking was instantaneous and the return to normal took time, but this also falls under writer's discretion. 

PLEASE NOTE THAT I REALLY DON'T LIKE SCAT OR WATERSPORTS. Cannot stress this enough. 

Our protagonist has been prophesied to die should Charybdis, the Living Whirlpool, remain chained to the ocean floor. So, what is a human to do to release a monster that eats ships whole on a regular basis? Attract the attention of another monster, of course!

Ending to you, gender of our protagonist to you, but I ask that the following two monsters appear and meet:

Charybdis: The Living Whirlpool. A beautiful woman who spurned the advances of someone skilled in magic (although some tales say a god, and some say it was a jealous Circe). So, she was cursed to spend eternity at the bottom of the sea, a massive monster. The only way she could keep from starving was to constantly pull food into her mouth by drinking the ocean, and because she never learned she was massive, she is likely unaware that her primary sustenance has been entire sailing ships.

Dracena: Dragon by day, beautiful woman by night (in the original myth, she was seduced and slain by a hero in her vulnerable human form, but let's ignore that). During the day, she rampages, causing earthquakes with every footfall, eating villages in her path, and killing anything and everything that approaches. At night, she is the world's greatest sorceress, able to break any curse but her own. She isolates herself as much as she can before morning comes, so her dragon form causes as little damage as possible. 

I'm good with just about anything, short of scat and watersports.

Poor Mia has been pushed around all her life, and has struggled to find her voice. This is made only worse by the unfortunate curses on her head: her size will vary with her self-confidence, and she has flawed immortality that lets her be killed, but she'll always pull herself together. The more browbeaten, the more viciously she's reminded of her low status in the world, the tinier she becomes . . .

Until Elena kidnaps her away from her life and family. Elena is aware of what this size curse is capable of, and, under the magnifying glass (or microscope if you let her get that small first), Mia is gorgeous . . . so, the lesbian sets out to increase Mia's confidence in herself, so that she can be the personal toy to a sexy, titanic Goddess . . . that's the plan, anyway. 

But yeah. Most (if not all- I'm not much a fan of male giants) of the story should be F/f- some setup with Mia dwindling away as she realizes how powerless she is, getting occasionally squished or swallowed, sometimes intentionally, sometimes entirely by accident, before Elena turns the story on its head, helping Mia get her confidence back so that the teensy lady can become a giantess that rewards the one who put her in power. However, whether Mia actually does what Elena wanted her to is your prerogative- she might have fallen too far in love to even dream about playing her role of cruel Goddess, she might still be so mad that she was kidnapped that she crushes poor Elena to bits. 

All you're really required to include is some violent/unaware in any parts pre-Elena's involvement, and Elena herself being relatively gentle (read : never intentionally hurting Mia) and gay for Mia, plus Mia's own size changing. I'd prefer no scat or dudes, as they're not my cup of tea. Beyond that, have fun.