To me, there are a lot of factors to determine when "average" becomes "large." I'm 5'8 and have 7c, which seems to be large on my frame. I've messed with guys that were 6'5 and had 7c and it didn't seem as large since they were so much taller than me.
For girth, to me, anything around 5.5 and over is what counts as above average.
