Bears are the most difficult animals to judge if you don't have a reference of some kind. Lots of experienced bear hunters have killed many "big" bears only to find ground shrinkage is a real thing. In Maine, where it is legal to bait we usually judge size based on the bait barrel. I once had a large bear walk behind a bait barrell and disappear into the swamp. About twenty minutes later a bear approached the bait on the trail i came in on. I tought it was the same big bear just circling around. I shot him while he was in the brush with a clear shot to the chest at about 10 yards. When we found him he was rather smallish and i vowed never to take a bear again unless I had some objective way of determining its size, Having said that, when you see a really big one you will know it. If you have to guess how big one is chances are it is about half the size you think it is. Been there, done that!!!