In all honesty I've only had two girls struggle with me and as I've said elsewhere I'm not exactly King Dong. One was a mature woman who said it hurt her in doggy, another was a Japanese girl called Ayako who said she could feel me in her stomach and pushed me back in missionary. Had a few put their knees in the way, but I'm not sure how intentional it was. Only had one or two girls who could get all the way down in oral, so we are talking less than 1% and in about 20 years since I've been shagging.
That said though I am what I would class as being above average, but not in anyway massive. In all honesty as I've said before I get off big time on dirty talk and I'm going to say "suck my big cock" as it just sounds better and whether or not either of us believe it, it's fun playing up to it. I have heard the opposite with men with big cocks who see dommes get off on small penis humiliation. Some women of course will say what they think the man wants to hear, I don't mind a bit of flattery, but there has to be some truth to it as my built-in bullshit detector will go off like an air raid siren.