So long as you go slow and use lube if necessary, large as in girth is OK, at least vaginally, for me anyway. But I find over 8" in length and depending on the position it can bang your cervix, which after a while is f@#!ing painful! Especially in doggy, he might have to moderate his thrusting, but in those circumstances it's probably best to go on top, so you can control the depth.
I dated someone who was so thick I couldn't get my (admittedly small) mouth round him, but so long as I relaxed and was warmed up a bit, the sex was fantastic.