You are wise to be asking this question now. It will save you lots of grief down the road.
You need a table that shows the capacity of the Heat Pump brand you are going to use. I have attached one below. The amount of BTUs that a heat pump can put out is directly related to the water temp coming out of the ground. You will notice that there are three temps given on the page.
At 68°F incoming water temp the 048 four ton unit has capacity of 54,1000 Btuh.
At 50° the capacity is 43,000
At 32° the capacity is 36,900.
So you need to know what the coldest the loop will get during the heating season.
Most ground loops are figured at 32°. At this temp the 4 ton unit only gives you 3 tons of heat.
Some people wrongly asssume that a 4 ton heat pump will give you 4 tons of heat under any condition.
This is why there are so many posts on this forum complaining that their heat pump is not keeping their house warm.
Hope this helps.