Screen size depends on viewing distance the larger the distance from eyeballs to screen, the bigger the screen should be.  I have a 23" monitor, if I back up to about a metre and a half, it's probably big enough, so under that it would be good.  I have a livingroom that is about 2,25m from the futon to the screen, a 23" might do, but 26" would be a lot better.  The bedroom which is about 3m from eyes to screen, the 32" screen in there is a good size.  However if you have a big livingroom that is 5m from seat to screen, that 32" would be way too small, but a 60" would probably do nicely. 

Measure the distance from the normal sitting position to the screen, view televisions from the same distance.   This can be difficult in some stores, where your normal viewing distance is longer then the aisle is wide.  There are two reasons for this, one is that it allows you to guage screen size, second it allows you to guage resolution, as viewing distance increases, resolution becomes less important.  This can actually save you money, in that a larger screen doesn't need a high resolution, but a smaller screen does.