Well, that depends on a lot of variables.
Men's penises have an average length of about 5-5.9 inches while flacid, and a girth of about 4. Of course this grows a bit while erect. And it's the erect messurement which usually *cough* counts for more. Usually with black men with the biggest lengths, then white men, then asian men.
Of course, there are mammoth men, right down to the micro-penises (did you know that's an actual medical term). In all cases of course, size matters. If it's too small sensation may be lost. On the flip side though, if it's too big, then your poor partner may not be able to accomedate it without putting themselves through a lot of pain.
The size of your gf/bf does matter aswell. If they have smaller anuses/rectums or vaginal canals it's to the benefit of a man with a smaller p***s. There are, believe it or not, a lot of men faced with a situation where their gf/bf always bleeds after sex, because they were so big (see, the grass is always greener).
Also, in a study of micro-dicked vs. differently sized men, it was found that men with small dicks (we're talking 2" or shorter) were somewhat more likely to have a more satisfied partner. Why? You may ask. In most cases these men were aware that they couldn't really deliver in the penetration with natural p***s department, and so put A LOT MORE EFFORT into their relationship, and the sex they had with their partners.
I think that might be the best evidence of all. Size does matter. But a lot of other things matter too. If you've got an awesome c**k, but you won't go down for your gf/bf ever, then you're a pretty shitty lover. c**k size only matters so much.