I would do one of two things.
1) Start with a heater value and turn it on to tstat control.
Place temp monitors or plastic bottles of water across the shop, as far away as you can get them. Check bottles for freezing, if needed, add more heaters.
2) do some basic calculations, but you have to measure areas and estimate R-values and pick an exterior temp. Use classic conduction equation, Q (heat) = A (area ft^2)/R (R-value) * (delta T) .....Q=A/R*(deltaT)
In English/SAE units, BTU/hr = (ft^2)/(ft^2 * deg F *hr/BTU) * (degF)
You have to do this calculation 6 times, to consider all 4 walls, floor and ceiling, estimate R values for each, concrete R = 1. I'd use something like an exterior of 20F although if you know it regualrly gets colder, then use that. I'd estimate ground temp as 50F. When you reach the total you will have a number of BTU/hr that have to constantly flow to maintain your temp. Can easily convert to kW if you are going to heat via electric. This is a quickie number and can't account for factors like north wind blasts and air leakage all which steal your heat faster than the basic simple conduction equation.
3) You didn't ask but: There's nothing wrong with getting say a 30K-45K-60K-80K BTU/hr natural gas heater which is way overkill for the "above 40F" you desire, but put it on a "garage" thermostat, it will run here & there, but only as needed. When you want to work or occupy the shop, you dial it up to 65F and you'll be happy in 15 minutes or less.
