You need to know the ceiling area, the wall area, the exposure, the infiltration rate, the R-factor of your insulation and the area and U-factor for the windows and doors...there are calculators that will let you input that sort of information that are free, but to get a better result, you need to often pay for a better answer. Then, you need to know your local degree-day heating requirements and your design temperature. Keep in mind that a slab like that will have a huge thermal mass, so any changes you want to make will require quite a long time, often days to make a radical change...it's not like bumping the thermostat up on a forced air system...or even radiators, you are trying to change the temperature of the entire slab. Generally, people will be more comfortable at a lower temperature with a radiant floor than other types of heating. MI can get frigid.
You will also need to know the spacing and length of the loops to determine how much energy transfer is even possible. You don't want it to require a super hot floor, as that's not comfortable nor is it very efficient. The slab will have issues if you try to get it too hot, and the pex won't like it either as it will be expanding and contracting more if the delta-T is high. Generally, the actual delta-T isn't very big in a slab, so that isn't an issue at all.
You need to look at what the minimum return water delta is allowed on whatever boiler you choose, as that can be radical, especially on startup from cold soaking. A condensing boiler can generally handle fairly cold return water, but you do need to keep that in mind.