Why these dimensions
Most 36 × 36 in bases give you about 2'-8" of clear floor once the walls sit on the flange. Ceiling averages 6.75 ft, so the raw room is 48 cubic feet. Tile, glass, and a cold start multiply that by 1.8, which is 86.4 adjusted cubic feet — inside a typical 4.5 kW chart. Insulate every cavity or the chart no longer applies.
4.5 kW at 240 V draws 18.8 A. Heating is a continuous load, so the circuit is sized at 23.4 A and landed on a 30 A breaker. If the generator you can buy is listed only for hardwire, keep the same wire and use a disconnect you can see instead of the receptacle.