What space is needed to hold 50 participants in a studio?

Well that depends on how extravagent you wanna be with your lessons. not much space at all is needed if you do an on the spot circuit type session - basically you tell them what the following exercise is going to be and they have all kit needed with them. If you wanna make it more fun and interesting maybe like an hazard circuit. The change is done once the hazard lap is completed how ever many times - do this in pairs one works on the hazards the other on the exercises and you'll need a space of roughly 15meters x 30meters and some imagination along with a bit of equiptment, (thats just an idea for you :) )
Smudgepti - Brit Army - Infantry - Mil fitness instructor