I have to tell you this works great. I know some of you doubt this but allow me to point out this is the same process the native ammericans have used to heat up sweat lodges for hundreds of years.
Obviously the larger the area you are heating the more rocks you will need, but I can tell you from personal experience that just a couple of rocks the size of two fists was capable of heating an area the sixe of a large 5-6 person dome tent.
A load about like what was originaly described[a couple of gallons] is what is normally used for such a sweat lodge, and will heat it up hotter than the average sauna by a great bit. Heating a small room would be no problem.

Obviously there is some variation with the type of rock used, and the amount of area to be heated, but the process will work. The ammount of wood involved is equal to a largish camp fire/smallish bonfier and it takes an hour or two to heat the rocks up so the wood used is an investment, but you would use more in an open fireplace overnight.

Just my 2 cents worth, but I would not have belived a few rocks could have heated an area and retained the heat for so long had I not seen it.