Id actually wager to say 2x2s are the least efficient due to the auto waste of 4 firewood for the fire on top, and that being a higher percent to the total of your firewood, would make it a fair bit less efficient per firing then a max 11x11 (I like my 4X4X4 but am going to do a big upgrade to 10x10x10 so I don’t have to fire it up for a long while)