I have a 5600 watt heater that can raise the temp about 10 degrees in an hour. Yesterday it was 23 outside and the heater brought the temp up from 34 degrees to 48 degrees in a little more than an hour. 5600 watts is 19K BTU so I was thinking something around that rating or slightly lower might be good.
I was thinking a little different....if you turned on a heater at fixed power, and then let it run until you reached a steady temperature increase relative to the outside.
I would expect that if you left that 5600W heater on, with no thermostat, it would heat the space well above +10°F. But if your goal was to turn on a mini in the AM to use the shop later in the day, then you would want to upsize the unit relative to what is needed to just barely heat it all the time.
And the COP is higher when these run at lower throttle, so upsizing the BTUs of the unit is an upfront cost that pays back later in higher efficiency (contrary to other kinds of heaters where too big lowers eff).
The higher BTU rating would, as above, also get you warmer faster, which is an advantage if you are not leaving it on 24/7.