I have problems sometimes with my boyfriend's knob!...lol..Anyways, I have certainly not gone through with a session before based on the bloke's size ( too big or too small), mostly just for safety reasons alone. If a guys knob is fuckoff huge, and I don't even have a rubber that looks like it will fit, then it's a no go. I've also encountered ones that are so tiny and skinny that I don't have any rubbers that would fit it, so obviously those are no goes as well...
All ladies are different, some can take more/deeper than others! I generally don't have a problem with well endowed gents
xx