What Mike said pretty much.
In tournament play back in the day, shooters were not supposed to be above 3/4". Any marble a little bigger than average might have been called a shooter. You will see that in some Marble King bags. The label will say something like "a shooter in every bag" but you may have to look really hard to find the slightly larger marble.
Nowadays people might say shooter for anything 3/4" or higher (but you can bet they haven't actually knuckled down with one of those really big marbles).