At first glance this seems to be a simple question which should have an easy answer. In actuality the answer to this question is much more complicated. There are two factors that must be quantified in order to determine if a particular masonry heater design will heat a defined square foot area. One factor, which is relatively easy to determine, is the maximum heat output of the masonry heater. The other factor, determining the heat load (loss) of a defined square foot area under the coldest conditions, is more difficult.
That is a loaded question, and you can get into a lot of trouble promising a customer to heat xx square feet. We have one heater heating a 2800 sq. ft. super-insulated house in a cold climate. An identical one was built in a milder climate, and is incapable of heating a 1000 sq. ft. "poorly insulated" house. "Insulated" is a misnomer, since leakiness is often a bigger factor than insulation. "Efficiency" would be a better term, or "low energy" or even better "low heating load".
You are better off to quote the heat output of the heater, and leave it be the client's responsibility to determine how much of his heating load it will cover. You can also quote some magic voodoo stuff about radiant heat. While real, it is much harder to quantify. The generally accepted maximum output of a large heater is 20,000 BTU/hr, or 6 kW. One of those cube shaped 220V shop heaters is about 5 kW, so imagine one of those blowing continuously. To get 20,000 BTU/hr, you need to burn about 100 lbs of 20% moisture wood at 70% efficiency. Typically, that will be two 50 lb fires per day. If you try to build a heater larger than that, you really need to know what you are doing, or be guaranteed durability problems. Some companies up the rating of their heaters by specifying 3 fires per day. So you'd get 20,000 BTU/hr with a 33 lb capacity firebox.
(Respondent A) is correct: The only thing you can safely guarantee is output. If the heater is planned as the major heat source, I ask people to provide heat loss statement to compare with heater's output. I also can give them rough assessment of heater's capabilities bases on the max output for a reasonably largest possible heater at 25000 Btu/hr, extreme cases, 30000btu hr, and on rule of thumb for heat loss per sq ft related to the estimated R-value of their outside walls. The numbers for this rule of thumb were given at least a couple of times in this forum in the past. Super insulated home (R30 walls +) will have heat loss of 10Btu per sq ft or less, while a century home without any insulation may be in 40-50 Btu per sq ft.
Respondent A (additional comments)
Using this rule of thumb, for a 20,000 BTU/hr heater: Super insulated house: 2,000 sq. ft. Un-insulated century house: 400 sq. ft. You also have to factor in location.