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You can either go for a 1 ton AC or a 1.5 ton AC depending upon the amount of light that enters the room, area and direction of windows, the floor on which your room is situated, number of people occupying the room and the number of lights.
As a general guide for moderate climates, a well-insulated room of 20 square metres will probably need a 2000W electric heater or a gas heater with 6 to 8MJ input. The same room would need more heating if poorly insulated; even a 2400W electric heater could struggle, and you could need at least a 10 to 12MJ gas heater.