Well, average is almost always considered something like this:
5.5"-6" in length, 4.75-5" in girth.
My girth is 5", maybe a bit more. When I push my penis down in measuring mode, I am 6.5"--although I know once I let go, my penis somewhat like "sinks" into my body and it ends up being more like 6 or 6 1/4". Even though the way I measure 6.5" is the 'official' way in a sense, I'm not sure if I should really consider myself that. Then again, it IS the "official" measurement. Oh well, basically I'm making a big deal out of it because technically that would put me in this in this gray area you're talking about.
Because of this, I'm technically "above average"--but in reality, if one had to place me in small, average or big, I'd be average. I don't feel there'd be a significant difference between my size and what one typicals sees as "average". It's like guys who are 7.5", most just round up and say 8". No one can tell the difference. I wish I could do the same and say that I'm 7", but since the proportion of my made up length is higher than a 7.5"er, I can't pass (except maybe if I told a very nieve and/or unexperienced girl that).
So basically to answer your question, that gray area is exactly that--a gray area. To a girl that's had sex with three guys who are average, this 6.5x5.5 cock is "kinda big". To a girl who's had sex with several guys, including big ones, might say "he's about average". For me, i'm not far from that area and I've never heard a girl say that I was big, because I'm not. I guess you can say that area is the TRUE average, since it's like exactly in the middle of the whole penis spectrum, while 5.5"ish is more like the COMMON size.