Catrike 700, Greenspeed GTO trike, , Linear LWB recumbent, Haluzak Horizon SWB recumbent, Balance 450 MTB, Cannondale SM800 Beast of the East
Frame size is the distance between the center of the cranks to the top of the seat tube (the frame part where the seat post goes in the frame). It used to be simple when all bikes were diamond frame road bikes but now you have such a variety of different frames that it is best to find a bike that has the same frame geometry and fits the person. Then make that measurement. I fit a 21 inch road bike but need a 19 inch mountain bike both with the standard diamond frame.