They are built stronger that the small ones, and some use plywood for the fins and centering rings. They range from 1.5 ft to 6 ft tall. After that you get into the high power rocketry, which use engines H and larger, and are built a lot stronger to stand up to high impulse engines. These rockets often have thicker tubes, that are sometimes made out of phenolic and other high strength materials, or reinforced with kevlar or fiberglass. They are usually larger than 3 ft tall.
Black powder is powerful for what it is, but there are other propellents that have more power.
They have 2-3 times the power as BP per pound. These are on the left of the picture. You can buy single use motors which you use and throw away, or you can buy reloadable motors. Reloadable motors are an expensive investment, but they pay off in the long run. They offer more prep fun than single use motors do, and if you plan to do a lot of flying, it is less costly.