They are quite large 22-25"+. They work well and I can build one from start to finish in 5 minutes. There is also the Cal Aqua Labs In-line 13mm diffuser.
For nano-med(50G) tanks I like to use diffusers to get a nice fine mist and the plants love it..
Your other option is a needle wheel pump, that has been a very promising way of diffusing co2...