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. I have flown several composite rocket motors. Also, I flew my Aerotech Cheetah on an Aerotech E30, F12 reloads, and some others I cannot remember. All of these engines were made by Aerotech. I recommend these engines to anyone who is looking for something different than the Estes motors.