your floor insulation has to be good to keep the cold off your feet...
spraying inside with a wood stove sounds dangerous to me..
(anyone else chime in on that)...because you have flame.
a shed that size you could have the wood stove but i'd use
a couple of those oil filled electric space heaters during spraying time. and use the wood stove for all other activities except spraying
a cheap souce of windows is old double pane sliding glass doors that nobody wants any more when they change them out
for the roof consider the snow load and if your going to use 1/2 inch ply wood you'd be better off with tighter spacing...
definately add a center cement foundation instead of spanning the 12 feet...just throw in a 12 inch sauna tube down to four feet.
i'd go 16 on center for the roof rafters to break the plywood better than wide 24" spacing.