Yes, size is important, but only to a certain degree.
Like I've said before, I've been with guys who were "bigger" than normal and didn't know what the hell they were doing.
I've been with one guy, who was relatively small...and that was the fastest I've ever finished in my life (5-10 minutes from start to finish).
So, while you may be big...you probably don't know as much as you thought you did...and leave a lot of women unsatisfied.
I'd rather be with someone who knows what he's doing than with someone who has a huge dick.