What would you use to measure a gym?

I would use a tape measure! Length times width will give you the area, or square footage. Then, mutliplying that number by the heighth will give you the volume. So that would tell you how much air needs to be moved with a forced-air heating and cooling system. You may want to measure something else, though, like cost in dollars per month (for a membership) divided by the time available to be spent there.