Zero sq ft would work just fine for a crossfit.
these are cash-cows. it's hard to turn a profit in a regular gym just selling memberships to cover overhead, and your PT sales are the gravy/profit.
reg gym = 10k ft at least, plus equipment that runs 200k min (lease or finance), plus your electric and water and insurance. you're lookin at least 60k/month overhead. charge 20-30$/month
crossfit gym = 1-2k ft.. as low as $.40/sq foot rent. 4 cages, 8 bars, plates ect... you can equip a crossfit place for less than 15k and just pay a couple instructors. charge 100-150$/month