How does the warmth spread?
Masonry heaters give off a little bit of convective heat, and a great deal of radiant heat. Radiant heat is a wave of energy that passes through air, rather than heating the air. Because of this it warms our body tissues directly, and this feels great. It lowers blood pressure and heart rate while relaxing muscles.
The question here, though, is how the heat will warm a home. The radiant nature of the heat impacts this in a couple ways. Firstly, it means the heat spreads through objects. To a degree (pun intended) you'll have warm tables, counters, chairs etc. Secondly in older, less well insulated homes, a heavy stove won't lose as much heat as wind blows through openings and general looseness in the home's wall system.
The Way of the Warming
Most Heating appliances heat a room up and then let it cool down again, so they are sized to raise the temperature from a low point to a high point. Masonry heaters are very steady, warmth is constantly expressed from the surface, A heater slowly changes the temperature of a space and excels at keeping temp steady.
Because the output is steady, constant, and radiant, the heat spreads through the house as far and as quickly as the envelope insulation allows it to. A properly sized masonry heater will first heat a "core" area around it's self, and once that's warm the energy from the stove will push warmth further.
Think of the home as a bathtub. The tub's drain size represents the heat loss of the home, and the faucet's Gallons-Per-Minute flow rate represents the heat coming from the heater. The right sized heater will overcome the losses of the "drain" and push heat (water) further up the tub (house) from it's self. The smaller the drain (heat-loss) the larger the space that can be kept wet (warmed) with the same heater.
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As always, call me anytime. I can clarify this answer or speak to your specific situation. 207-766-6003
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Eric