I tend to define big carp as anything over 15 lbs. Granted, this is an arbitrary number, but a 15 lb carp out here is roughly a third bigger than your average fish, so it feels right. Plus, a 15 lber is always notable. I hear a lot about "big carp" from gear guys I run into, or random people...I guess I spend a lot of time asking "seen any carp around here?" so I shouldn't be too surprised when these "huge" carp turn out to be normal 7-10 lb fish upon exploration. Most people simply haven't held a lot of 10+ lb fish...heck, most fisherman (myself included prior to my carp days) haven't held that many 10+ lb fish. Word of advice to any serious carp guys...carry a scale. I only weigh fish I think will hit the "big" limit, but a scale is easy to carry and impossible to refute. Carp get big, and it is nice to know just how big that fish is...and difficult to figure out without a proper scale of reference.
One thing to look at...the size of their tail. Big carp have big rudders.