I mean, while as in general I like the idea, in practice I’m pretty burned on F/m content, so if I was to write it, I wouldn’t want to do that, either.
There’s a couple of ways I think could get around it (well, outside of it being F/f, which while I’d like more for obvious reasons, probably isn’t what you’re going for): the first is simply… skipping over the F/m parts. Start after the guy is giant again, or as he’s grown back, and feed the SM backstory organically as they interact; Missletowe, back in the old forums, had a story done in that fashion, but that was a pure revenge trip where the guy had been abused until the shrinking machine got fucked up by lightning and flipped the script on his captors (one of who shrunk to nothing).
The other way is… well, don’t have the interaction at all. The guy is a little statue, or something: can’t move, can’t talk, but he can perceive the world around him, to some extent. It works really well in a magic setting, because then the woman thinks he’s a fairy or something, which is something she could naturally expect rather than him being something bigger and just compacted, and she coos over the ‘little guy’ all the time while trying to get him free, and giving him a good impression from his eternal prison… and then when she finally releases him, the giant demon lord sealed away millennia ago, or whatever, pops out, completely unexpected on her part, with neither of them having properly interacted before, but both with positive impressions of the other.