That sounds about right. As a rough rule of thumb, mortar-fired
firework shells go up about 100 feet per inch of diameter. Many
small-to-medium municipal shows don't shoot anything above 6" in
diameter, because larger mortars have to be buried for most of their
lenght, while 6" tubes (or smaller) can be set above ground.

Larger shows will include 8-12" shells, and even larger shells are
available (though they don't see a lot of use).