Sony 56inch Plasma, with a HTW840 5.1 system, only set you back around �5k but worth it.
Or, hold your arm out straight, make a fist. if your fist is bigger than the screen them it needs to be bigger. (remember how far you will be away from the screen)
I see you have two good answers, but is your room really 12m2 (square metres) or is it 12m3 (cubic metres)? If it is 12 square metres then you have one big room that only ppl in USA usually have.. unless you're from a wealthy family and live in a mansion.