The way I see it, it's kinda like Mr.Hardcock said, it's the overall package.

If someone has less than 7, say, 6.5 to 6.75, but has a girth of 6", that's surely gotta be classed as big as well. You suggest that 7 is pretty much where big starts, but some people have 7 inches length with less than 6 girth. Girth is so important and in many ways seems overlooked and overshadowed by length when considering what is 'big'. Length seems to be the thing that most people qualify as big, yet forgetting to factor in the ass and pussy widening capability of cock fatness. Having said all that, I do realise there are some people where length really is more important than girth. But from what I've read, girth more often than not gets the higher vote.

However, if we're purely talking about length only and not girth at all, then yes, I would agree that 7 is probably the starting point.