a) If you’re speaking of aesthetics then this is exclusively to the taste of the beholder and hence fairly meaningless to try to determine as a general rule. People will have different cut-offs. Also, flaccid to erect ratios, length to girth ratio and all that come into play, not just a single number.
b) If you’re speaking of (sexual) functionality then this is a compromise of the preference of all parties involved and hence fairly meaningless to try to determine as a general rule, as some like it bigger than others.
c) If you’re speaking of day-to-day functionality then this is down the the individual and hence fairly meaningless to try to determine as a general rule, as some guys won’t mind massive swinging salami whereas others will find it a burden to cope with during simple tasks like walking.
Personally, in those three categories, I:
a) think that on my fairly large body size, my fairly long penis looks average and would prefer it to be a bit longer, mostly in the flaccid state but also somewhat in the erect. But I’m more interested in girth, still.
b) find that some of my length is wasted anyways as at least 2-3 inches hang out of my lady during deepest penetration. When she’s pretty fucked up and stretchable, I can go perhaps an inch or more further in with no pain but feel a definite wall and resistance to going further (plus some pain with too much pressure). I would prefer to be balls deep, so to me I’m actually past the “too long” point in this category (at least specifically for her). It’s something of an ego-boost for me (and her, in fact) though, so it’s a strange dichotomy there.
c) sometimes catch people taking a glimpse at my package which doesn’t really bother me. It can be a little distracting at times. I have never really found any difficulty in day to day activity due to the length of my penis. For example, it has never got caught in a lawn mower or meat grinder.