It's not how big it is, but how you use it. A big dick might seem like a blessing, but if you just go in and hit everything without any idea what you're doing, you aren't going to please her much. However, if you've got a slightly smaller one (obviously not too small) and know what you're doing, the result is great sex. It's like the difference between firing a mortar at the enemy and firing a guided missile. A mortar round might hit, but a guided missle probably will.
Of course, if you're both well endowed and know what you're doing, all the more better for both of you. And if you're too small and have no idea ... we'll you're fucked (and not in the literal sense).