That might work. You could use masers, but I think, in the interest of costs, a cavity magnetron and waveguide would be a better choice for generating the microwaves. You'd have to build a large metal cage around the area with the gaps being smaller than the wavelength to make sure you weren't heating anything unintended or causing electrical interference to nearby devices/circuits.

There'll be the problem of the resultant interference pattern in the--what should we call this?--microwave sauna, creating cold and hot spots. You could set it up so the magnetron and waveguide move around, but that's a failure point centered on the most expensive components. It would probably be wiser to put a rotating platform in the sauna to move the subjects through the bands of positive and negative interference for more even heating.

You'll probably need to have controls for power level and time. Can't have the thing on full blast constantly or people will get a little too warm. Those controls will have to be on the outside, so they don't receive interference from the microwaves. Also, it'll be really important to educate people not to bring metal with sharp edges into the sauna, otherwise the potential difference between the magnetron and the point/edge of the metal can overcome the electrical resistance of air, creating a plasma arc that could short the whole thing and electrify whatever the metal is connected to. It needn't be a long list of banned items, just simple things like cell phones, bank cards with chips, zippers, keys, jewelry, dog tags etc.

One of the coolest things about this solution is based on the fact that corn kernels have the moisture to absorb enough energy from the microwaves to make steam and cook internally until they rupture, forming a tasty snack. You could totally put a button on the control panel for making popcorn.

Being around microwaves like that has been proven to melt chocolate bars in pockets, though.

Ra's al Ghul would like a word with you

We haven't heard any complaints, though! Just the distant sizzling sound of contentment.

Given that they have to be a distance apart relative to the wavelength they're emitting, I'd guess about 10 times larger than a single emitter with the same output. However, because it's able to create beams with cross-sections similar to the profile of its targets, I'd argue it could just have a lower output and resolution with similar performance to a single emitter for half that. Those are wild guesses. Hopefully an engineer is lurking around here somewhere.

With respect, I fail to see how that advances shareholder value. How would you even build a recurring monthly fee into the use of clothes?