There’s a list of books, comics, and manga related to size being semi-regularly updated by the community on DeviantArt by user Phoenix-Firemage. I’m not sure how much of it has M/m, but there’s definitely M/f content.
I don’t know if they’re on this site, too - I’d mention them if I did - but they also have separate lists for TV shows and movies, too, if you’re interested!