Man, I have experienced this. It takes a while to become wet enough, aroused enough, etc.
I haven't had this "problem" in a couple of years. Now when I think about him (the BIG guy) it turns me on to the point where I know it would be NO problem now!
Just find a way to become aroused enough through foreplay. It will help 'big' time!!
Oooh. I just thought of a really great position. Do it doggy style, only, instead of him moving and thrusting, you move back on him. I loved that position with my big guy. Another thing. . . have him enter into you inch by inch. Slowly in and out. V E R Y S L O W L Y. Eventually it should be no problem!