I don't have a lot of experience here, but I am more than happy with the guy I am with. As long as he's not on top it does the trick. Sometimes when he is on top it gets in too deep and it hurts, but I like to be on top and in control, and that is great!
i have a preferred size, but as cliche as it sounds.. for me at least, its the motion of the ocean, not the size of the wave.
It doesn't matter nearly as much (to me) as how sensual and open you are. I think that even if you have a 10 inch cock, (scary) or a 3 incher (tiny) if I really care about you, we can make it work. Beyond that, I will say that different posititions accomodate different sizes in different ways. So play around with your positions, and even if your penis is a little short, you can find a few 'staple' positions that will please you and your partner.
The answer is, it depends on a lot of things like people mentioned. I wouldnt stress to much about it. If you want to be a good lover, then work on being a good lover (and that's a task in itself). You can't really do all that much about your size. I have a smaller than average penis and it has never really effected my game- I get laid pretty often, and usually lay out at least a couple orgasms on a bad day. But that's not what its really about anyway. In my old age (24) I actually care about the emotions involved as much or more than the pleasure.