To calculate a well insulated room for electric heat try calculating the width X length X height X 1.5 divided by 250 watts per foot of heater. This will tell you the minimum heat required to heat that room.
To calculate watts multiply the number of feet long the heater is times 250 watts per foot to find watts of that heater.
To calculate how many watts you may put on a 20 amp 240 volt branch circuit then you would be able to install 4800 watts per 20 amp branch circuit @ 100%. I advise not to exceed 80 % load on each 20 amp branch circuit.
Hope this helps