There’s a calculation for the size of stove for a room at stovecentre
A typical example of calculating the heat requirement of a room is:
If Measurements are in feet
Height x Width x Depth/500
If measurements are in metres
Height x Width x Depth/14.5
This will give you the number of Kilowatts required
If you multiply this answer by 3,420 this will give you the Btu’s required (British Thermal Unit)
And a whole load of sites for ideas
And the usual retailers as per Google