Yes. And other things in the world matter too.
The most my girlfriend has said about the issue is "size does matter, [but] you are perfect". She says it fits her inside her perfectly. And I can tell she loves the guy attached to the dick (me), more than the dick itself, however big or small it could be.
She tells me all time that it's the perfect size for her, which is good because she has expressed interest (mild awe) in rumors about well endowed guys. I felt a little bad about that, but I understand that it's just awe, like when you see a girl with a perfect rack, it doesn't make much sense to try to pursue that girl just because of what a physical attribute alone.
I like certain body types, usually fit or slim, little or no belly, nice compact, round butt, b or c cup breasts. A pretty face.
These are just things i find attractive. My girlfriend has none of these, except for a very pretty face I have fallen in love with. Though she is very soft around the middle, has small breasts, doesn't look like the perfect girl for me, ect.., I love her and wouldn't ever leave her just because I could find someone with a nicer body.
I'm sure she could find someone with a better dick than me, but I know she wouldn't leave me to experience one, or like me less, if we were apart and she had one.
So does size matter? Of course it does.
It matter if the relationship is just about sex, but even then, a big dick might not make great sex.
How about in a close relationship? How much does it matter?
Well, how much does it matter how sexy your partner looks? How long they can go for in sex? How many times they can make you orgasm?
I'm sure they all matter a lot, but there is usually something that matters a lot more in a relationship, and that is how much you love them.