You need to figuire out what size pipe, tube diameter you plan to run for your system. This is really the determining factor of how big or small you can build. The smaller the body of the cyclone the higher internal velocities it will have making it most effective. Steffens chart is a simple way to size as your inlet and and outlet pipes "without" modification drive the diameter of the body. Say your are using 2" pipe, for instance; so you have 2" going and 2" going out. So your body diameter can only be 6" any smaller and your outlet pipe will interfere with the inlet pipe and it wont go together. On my systems I have standardized our 4" body filter with 2" port pipes. However we form the inlet pipe so the center pipe clears it. I use this on both small and large engine machines. For the final gas dryer series system I use 3" body filter with 1.5" ports built the same way; However, because this is more restricting I may go to 4" body filters with larger ports.
So you first need to figure out what you think is going to flow best out from the gasifier to your application and then build the cyclone using that same size porting with the body diameter that will work.