If you feed the sauna stove from inside the hot-room, it’s a good idea to leave the main hot-room entry door up from the floor approximately 1″ so you have roughly 30 square inches of space to allow air to get in to supply fresh air to the Hot-room. The stove itself (set with the air control set to full open) only uses 4 square inches of air. The rest would be to provide plenty air for users to breath. Some people even have a floor or wall vent that’s adjustable to provide more air.
If you load from outside, then you don’t need as much space under the main entry door.
Daryl