Most girls are happy with average until she has been done by a man with a bigger sized one.
In America it is likely 1 in 5 girls would at some point in their life be done by a man with a penis that is longer than 7" and 1 in 20 would be done by a man whose penis is 6" or bigger in girth.
20% of girls will like it, 30% of girls will hate it and 50% won't care.
As a guy with a bigger sized one and done many girls.
Is it important to be big? Yes if you are in a longer term relationship with some bigger sized girls ie she has a big butt and large hips as well as thighs. Some girls that are taller will need a man that has a longer one and some girls that are bigger sized you will need to be longer for access reasons.
Being long may or may not be a good thing, not every girl likes it and like a guy with a 5-6" one during sex I would also be attempting to put all my 8" into her vagina both for pleasure and for ego. Not all girls had liked it.
Thickness? It depends on how it is introduced into her vagina. Also it makes a big difference if it is being introduced into a ONS/FB vs a GF/fiancee and yes mine once introduced into a virgin woman on her wedding night. Some like the thickness, some love the thickness, some didn't like it the first time but liked it later on and some hated it and never wanted it again.
The answer whether does it matter? Yes it does matter as you can't be too small or too large.
Some girls are size queens or just like the feeling of being filled up completely to the point where they're about to burst. Most women seem to want around average size. Altho, being just a bit bigger can never hurt.
I've said this a lot so far, but guys, if you feel like you're not as big as you'd like, start jelqing. It yields quick and permanent results.
Scroll Down to Read Other Opinions
What's Your Opinion? Sign Up Now!It matters to an extent, but 5-6 inches in length is fine. The vagina is only so large. It can expand, but it isn't infinite in space. If you genuinely feel your size isn't good enough, then get better at pleasing said woman outside of penetrative sex.
Most guys have around 6 inches.
There's lots of other factors that can determine whether or not you're a good fuck.
I think most women like an above average cock, but it's not totally necessary or anything.
We all want more of everything.
More tits, more ass, better face etc.
Yes and no! Some girls need a huge cock but that's maybe 20% of girls. I have a basic 6.5 cock but it curves so much that it hits most girls g spot.
That size is plenty for the vast majority of women.
Sexual Relationships 
Sexual Health 
Most Helpful Guys