Both the Greater Amberjack and the Lesser Amberjack are good eats. The minimum size limit for a Greater Amberjack is 28" to the fork, and you're only allowed one. The Lesser AJ has a slot size of not less than 14" or more than 22", and you're allowed five. The lesser AJ always seem to be that size, and when they're around they do turn out to be Lesser AJ not small Greater Amberjack. But it still pays to know the difference if you run into a picky FWC officer. Generally speaking if you see something looking like an Amberjack it's legal to shoot, not often you're going to run into the forbidden 6", if you do it will usually be a Greater AJ that's just short of 28".
The main difference between the two amberjacks is the greater amberjack is longer than the lesser amberjack. In fact, the greater amberjack is four times longer than it is deep (side height); while the lesser amberjack is only about 3-1/2 times longer than deep, not counting the fins. If a ruler and calculator aren't handy and you really want to know, lift up the gill and count the finger shaped projections from the first gill. If it's a greater amberjack there are about 20 of these; in the lesser amberjack there are about 25.