So, you are referring to "hot" composting, which is an accelerated process that generates a lot of heat. I experimented with heating my greenhouse using this method. It takes about about 4x4 ft of space, and the big problem I had is that you have to tend it all the time, get exactly the right mix of ingredients, and then, in the dead of winter, when you need to feed your pile, where can you get fresh grass cuttings, leaves, etc? In order to generate enough heat to warm a greenhouse, that pile has to be going ALL the time. You will wind up with completed compost in just a few weeks, at which point the pile will "die" unless you get a lot of the material out and feed it with fresh. The fresh material can't be frozen (i.e., from sitting outside waiting to be used) when you add it.
So, lots of space, lots of time and energy, and lots of storage for extra materials. But it DOES work, if you are devoted enough.
I wasn't, of course, and that is why I have a really nice blue flame gas furnace now!
Cheers,
Carolyn