change all the door gaskets, and look it over for leaks, especially where any linkages, etc. go into the stove body for raking, door handles, glass, etc. We had that problem last month with a Newcastle. It was cooking the owner out of the house and they couldn't turn it down enough.
it needed new door seals, new glass seals, and the raker rod hole in the side of the stove, was leaking air big time. It would run with the draft completely closed, just off the raker rod leakage.
we replaced all those seals, and I made a gasket for the raker rod hole, and now it's running fine and can idle right down to barely a simmer
if you "must" try a smaller stove, get a Harman I on the used market, they are around for about 1/4 price lately- or try one of those small potbelly RR caboose stoves, just realize if you trade down to a small bucket-a-day potbelly pot size, you'll be filling the stove more often. If you buy a used stove, make sure the firebrick or liner is good. There's a ton of old vintage stoves out there now, with missing burned out liners and no firebricks.
an old style mini 2-lid cook stove would heat that space just fine