Mike:
The outdoor wood boiler is a wonderful thing....providing you don't mind cutting firewood. I cut my wood 24" long as that is what fits in my splitter. The firebox is 44" long - when I load wood I rake the coals to the front and throw the wood in the back.....I load 3-4 pieces in the morning before I go to work, and another 3-4 pieces at night before I go to bed. It takes about 6 cords of wood to get me through a winter. I have a 16x18 carport for my woodshed - if I can get it full that is about 3 years worth of wood......it is best to let the wood season a year before you burn it.
Although the radiant floor heat in the garage is nice - it isn't the only way to use the heat from the boiler. If you have forced air heat in the house you can install a heat exchanger in the furnace plenum and have the thermostat turn on the blower when you need heat.....that is how our house works. For your garage you can install a heat exchanger and blower.....kind of like the electric or gas furnaces that hang from the ceiling - but it uses the hot water from the boiler...…..you can also use the old cast iron radiators.
If you have plenty of woods and can harvest the wood from your property - this is a good system. I have trees dying and falling down al the time - I rarely have to cut down a healthy tree for heat. To keep the property cleaned maintained I have to get the dead and damaged trees cleaned up - I might as well use the wood for heat rather than just burning it as waste in a brush pile.
Cutting and stacking the wood is a lot of work - it is good work and helps me keep in shape....and it is more fun than going to a gym.
Some folks have outdoor boiler that burn rounds bales - the end of the boiler opens and you put the entire round bale in! If you have a tractor and bailer this might be better than wood!
This guy does a good job explaining the good and bad. He splits his wood too small - I only make it small enough I can lift it without hurting myself.
https://www.youtube.com/watch?v=q2QTK66lJJA