Large bears generally have these traits...
1) Low slung bellies
2) rear legs look small compared to the overall size of the bear.
3) short stubby ears
4) waddles or rolls back hips
5) if you are bait hunting, you outfitter should have a measurement device next to the bait (usually a log or by barrell height). If he looks as big as the 55 gal baot barrell, shoot him especially if it is a multi-bear hunt and it is your first tag.
Here is what a big bear looks like... notice the round head, stubby ear and small in proportion snout...
Here is what a small bear looks like... notice the large ears and the longer snout...
Then again, if you are asking these questions about field judging, I am assuming that you haven't shot many if any bears. Remember it is about the experience and the hunt, you should be proud of any mature bear.
SA