A room measuring 20' x 20' = 400 square feet
400
144 assuming no waste.
26.66
220
20 Sqft.
340 square feet
15 x 20 = 300 square feet.
5000 BTU's should work for a room that size.
10 ft by 10 ft --- a 4 ft baseboard heater will work. But a longer room may need more. 10 ft by 20 ft would need an 8 foot baseboard heater. etc.
8-ft x 10-ft = 80 square feet If you mean "20 square foot a box", then you need 4 boxes.
10 ft x 20 ft room = 200 square feet