Hey Dave- re. the garage-shop, I agree with adding an extra 6' to make walk-around space.
Make the ceiling AT LEAST 8'. Go 9 if you can- that'll give clearance for a pair of inexpensive ceiling fans. You'll also want an exhaust vent for when you're running welders, grinders, or have some of your fire belch smoke into the room.
Rather'n heating part of the floor, heat the WHOLE THING. Half-heating the concrete floor will make the air feel cold, and it'll put the concrete under strange stresses
If you're planning on a woodburner, get one with a water-jacket, or better yet, get one of those fancy microprocessor-controlled self-lighting outdoor furnaces, and mount it (and your woodpile rack) under a carport, to keep the snow and ice off your fuel. Warning- you MAY like the outdoor furnace so much that you'll want to tap it into your house furnace and save utility bills...
To heat the shop when wood isn't convenient, install an ordinary, but large water heater (My neighbor uses a 40-gallon on Natural Gas and a small electric circulatory pump to heat is 28x28 workshop, and with R19 walls and R23ish ceiling, the workspace is T-shirt 'n underwear warm during -20's weather, just as long as you've got shoes on... without shoes, your feet get too hot!