What Output Gas Or Electric Fire Will Heat My Room?
Typically most Gas fires have a heat output of 3KW, where as electric fireplaces have a heat output of 2KW. Gas Fireplaces can have heat outputs up to 5KW, where as electric fires are limited in output as they work using a standard electrical plug 
How do I decide on output?
1. To work out how much total heat is needed in your room you need to work out the cubic area and divided by 14. Hence this is the height x width x depth (in meters) and the total should be divided by 14. This will give you the total heat needed in KW Some firms recommend that you allow additional heat output if your room has a large number of windows (as this created heat loss) 
2. Essentially the most efficient way to heat a room is by using a modern central heating system (this is the cheapest). Fireplaces are always arguably less efficient than central heating systems 
3. A 3KW average gas fire will be sufficient to take the chill out of an average size room and heat it in about 20 minutes. Electric fires are restricted to 2KW heat outputs and can take the chill out of a room much faster as they work using a fan assisted heater which blows are into the room 
4. A glass fronted gas fire (often referred to as high efficiency fires) are up to 90% efficient and have much higher heat outputs. These fires can heat a room up much quicker and are more efficient, which makes them feel warmer as there is less heat loss up the chimney