It really depends on the game. Loli/Shota—usually an average-sized dick, but for everyone else, a fucking massive schlong . Especially if the game's atmosphere/theme isn't entirely realistic or is even exaggerated or cartoonish. In that case, I love it when MC wrecks the insides of all the characters with his cock.
You reminded me of something I don't like NTR games where, for some reason, the MC always has a small dick. I've always found that unnecessary, i mean, it can still be NTR even if the MC is well-endowed.