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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.
According to this common but somewhat inaccurate method, you need 1 ton of air cooling capacity for every so many square feet of living space. While there's some dispute over the exact amount, an often-used amount is 600 square feet. A 2.5-ton unit, then, theoretically would fit a 1,500-square-foot home.