There are several things you should look at; the ears on a large bear seem small even though they aren't the head on on a big bear has more muscle mass giving it a larger profile. Their neck and shoulders have a bulldog like appearance, they look like their head is attached to the shoulders without a neck. The belly will look like it's about ready to bottom out and have short legs; where a smaller bear will look more leggy without the large barrel.

Bear Hunting in Alaska by Tony Russ has an excellent chapter on judging grizzly, black bears have most of the same features.

Good hunting.