I condider myself a big game hunter so i try not to shoot anything to small so they will grow to be a big bull for someone else. I consider a 5 or 6 point bull big enough to shoot. But a big bull would be a six point (most of the ones that are symmetrical and have a decent spread will make you crap your pants to see in your crosshairs) and they usually score 290"-310". Mass(or antler thickness), length of points, and spread will effect the score by making it bigger, but its hard to field judge them unless you sit and study pictures for a while and get out and see them all the time. ANY 6-POINT BULL IS A GREAT FEAT TO HARVEST, and almost every hunter will look up to someone who shoots a bull with the age and health to grow that 6th point...especially if it makes it over the 300" mark. The biggest bull i have ever seen in the wild was bow hunting last year, i would have guessed it to score about 360-380 and i have never seen another bull like it...too bad my bow wouldnt shoot 100 yards!
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