The heat load of the room is never calculated as function of it's square feet of floor area. In a basement it's a function of the square feet of exterior wall area above grade & R value, as well as the square feet of wall area below grade & R value, amount of air leakage, then the floor area and IT'S R value, etc. The heat loss/square foot of floor will vary widely- there is no magic ratio to use.
Once you know how much heat you need to pump into the space, then you can work on how many square feet of radiant-wall it takes to put that amount of heat INTO the space. Radiant ceiling would be more comfortable though, and would give you a larger radating surface to deal with.
Use an ACCA Manual-J type calc to come up with your heat load as a starting point.