Unless you want to completely 'wing it' and blow some cash by undersizing, or sometimes worse, oversizing the system or even mismatching components that will never work quite right together, you need to do some heat loss calcs. Too big a boiler for the system/heating load will short-cycle and burn itself up, and waste fuel. Poor design can cause a 92% efficient boiler to run at more like 80%. There are some great free heat loss calc tools available online. If your shop loses too much heat, and your trying to keep it at 70°F the slab will be so hot it will be uncomfortable. Just to name a few of the issues.
What is your target shop temp?
Design temperature for the region?
underslab insulation? (not a yes/no question) just how much.
slab perimeter insulation?
auxiliary heat sources?
Solar insolation? No/lots of windows?
insulated bay doors?
Insulation in walls/ceiling? Remember, code minimum is just that.
excessive groundwater?