Penis size correlation to ED?

Is Erectile dysfunction more common in large penises? I am 23 years old and am between 8.5" and 9" erect. I have no problem achieving an erection, but I can rarely keep it for more than a few minutes without constant and direct stimulation. When I masturbate I have to put pressure on the blood vessels on the base of my penis to achieve a full erection. It is difficult to do that while have sex with a woman, though.

Gorkahn