You might want to consider in-floor radiant heat...much nicer, and it doesn't take up wall space. The amount of heat you can get out of the radiator depends on the area, the incoming water temperature, and the flow. The manufacturer's specs should give you that info.
If you search around, you can probably find a heat load calculator application on the web and figure out what amount of heat you need in that room. WIth the high ceilings, a radiant floor will make it much more comfortable at the same air temperature.
Then, match the radiator size to the required heat load.