I've never had one that's been too thick (so long as I'm warmed up...), but too long definitely, especially in doggy. If this happens I move down into prone bone, which helps. It can sometimes happen in deep stick, too, but cowgirl, missionary and spoons are OK. I think for me it's any cock longer than about 6.5-7 inches erect (as measured correctly, i.e. along the top).
I've got a low tolerance threshold for my cervix being banged - it's fucking painful!