The sizing thing reminds me of a game I've funded on Patreon for quite a few years when funds allow, Breeders of the Nephelym, They've had a similar thing for years now with actors, they separate it by "Giver" and "taker", basically top and bottom I guess.
Essentially a massive character and tiny character cannot get it on, but a big and small has a few options, normal (the average medium size) has the most options, it's in a similar way to what you're going with, positions have an upper and lower size allowance.
You can find the charts on the games Wiki if you want to see a reference that's far better than my terrible attempt at remembering it off the top of my head.