flashpig.....I've never measured any that I've encountered so in all honesty, I couldn't tell you what, if any, size would be too small. I guess girth is as important as length, so relatively short and chubby ones could do the job as well (if not better) than longer skinny ones.
As almost everyone else has said, it is definitely what you do with it that counts. I've encoutnered men who have 'whoppers' and sometimes they act like they're gods gift and that you're 'lucky to have them' and so make very little effort to please a women. Regardless of the size of their penis, that would mean lousy sex. Cliche time again but if you are close to the person with whom you're having sex, and have a real intimacy, then size just won't be an issue, let alone a disappointment.