The required power of heater depends on size of room, its insulation, outside temperature. The following table gives recommendations what power depending on the size and height of the room with standard insulation best suit you premises. If the ceiling higher or insulation is poorer, it is recommended to choose bigger power heater.
 

	Mild climate (temperature seldom drops below - 200C) 60-70 W/m2	Normal/cold climate (temperature often drops below  - 200C) 70-85 W/m2
Heater power, W	Room size, m2 (ceiling height up to 2,4 m)	Room size, m2 (ceiling height up to 2,4 m)
400	3 - 7 m2	3 - 6 m2
600	8 - 10 m2	7 - 9 m2
800	11 - 14 m2	9 - 11 m2
1000	14 - 16 m2	12 - 14 m2
1200	17 - 20 m2	14 - 17 m2
1400	21 - 25 m2	17 - 21 m2
2000	26 - 34 m2	22 - 29 m2
2500	35 - 40 m2	30 - 35 m2

Also you can calculate your required power by pressing here...