For me "big" is all about the proportions with girth included! I think a very thick 6" is be bigger than and average girth 7". Long dicks are just long... add girth and they become "big"!
7" or over in length is getting long. 5.5" or over in girth is getting thick.
One dimension without the other will get mixed reviews. If you have exactly 7x5.5, I think many would call you big. Big, to me, is just well above average. Maybe +1.5 or 2 standard deviations above the average?