Each time the boiler comes on, it isn't that efficient at the beginning, and then, some of that heat goes out the flue plus, stuff cools off in between firing cycles. If your 200K boiler runs five minutes an hour, 200K/12=16.67K btu going into the structure. A 16.67K boiler, running 100% of the time will keep it the same temperature, and be more comfortable without peaks and valleys in between cycles. It will also probably use less fuel in the process, especially if it's a condensing boiler, as the return would be cooler, making it more efficient (pulling more energy out of the burner through the condensation). IOW, a huge boiler, running a short time will spend more of its time bringing things up to temp, and be less efficient in the process versus one closer to the actual need that is running steady state.
Many of today's condensing boilers are way more efficient than the older style, over-sized monsters and can adjust to the need.
An in-floor, radiant heating system is horrible at setbacks and recovery. The slab doesn't like super hot inputs, and the thermal mass just doesn't respond quickly to a viable inlet temperature for radiant. A cold-soaked concrete slab could easily take a couple of days to recover. If the shop ever could run without heat for awhile in the winter, you'd have to consider using antifreeze in the water. This will make the heat transfer less efficient by about 10% or so.
Dana is more up on what is a good way to do a heat analysis. Since you don't have an existing system where you could use actual fuel input to calculate, you have to go by the old-style, by using insulation, windows, exposure, and heating zone (design temperature).
The spacing of the pex and the resulting maximum heat transfer will need to be adjusted based on how much heat you need to transfer to keep the temperature of the water at a reasonable level without a lot of striping of the hot/cold areas. Keep in mind that a single loop shouldn't be much longer than 200' or so so that the returning water is not so cold that that area doesn't get much heat. That can be critical if you use a more conventional boiler as those will be destroyed by condensation since they are not designed for it and may need some bypass to keep the return temperature above the condensing range to protect itself, making things more complex. In an area that large, you will need a manifold. You may nor may not need zones, depending on if there's any partitioning or other needs.