According ACSMs Health Fitness Facility Standards & Guidelines 50sf per person is recommended. That recommendation was made for “aerobic dance” and allowed participants to move 8 steps forward, back, right and left. Based on your cardio requirement 50sf would be safe, but you could probably go to 40sf based on the movement style and the experience of the participants. Don’t forget to account for space in the room that might be taken up by equipment that is stored (balls, steps, mats, etc) or “dead space” (counted in the sf but not an adequate space for a particpant to us). Best of luck!