First, I am not sure if this was already suggested by someone else but if so, I didn’t saw it while checking a few minutes ago.

Second, I have literally no Idea how to code something like this or how much work this would need so I would like to apologize in advance should it be too much.

I would like to suggest that during sex, the size of the participants influences the efficiency of all actions in which a person is penetrated.      

For example, a point system could be used where each size category is worth two points (from Petit =2 to towering = 10) and one additional points for large or one less for tiny genitals. During the sex-scenes it could give something like 5% efficiency bonus per point the penetrating person is larger or malus per point the penetrated person is lager.

Also, it could be made so that when the penetrating person is more than 5 points lager it’s counts as punishment and with more than 8 it becomes impossible or starts to damage the penetrated. While if the penetrating person is too small (maybe also 8 points) it loses all effect on the penetrated person. 

Additionally, it would be nice to have more genital sizes as well as different genitals for females (just like the male variants). If for male and female the genital size/capacity  would be possible to choose with more variation (e.g., tiny; little; small; average; large; huge; gigantic*) it would also be possible to be a bit more divers with the size and how it works.

 *I have no good idea for how to call last genital size.