Depends how old and drafty the room is, and what temperature you want to maintain. If you have an old pre 70s house, a rule of thumb is 100 Watts per square metre to achieve room temperature. For a newish or well insulated house, use 50 Watts per square metre.
At 5x8 foot, this works out to 3.7 square metres, requiring about 400 Watts for an old house, or 200 for a new one. Most heaters will be far larger than this, to allow the soace to heat up in a reasonable time from cold.