The short answer is if you have a small room and your sitting close to the TV, then you need a smaller HDTV. On the other hand, if your room is large and your sitting farther back, then you can go with a larger HDTV.

Rule of Thumb: Measure the distance in feet between where you want to place your HDTV and where you will be sitting. Now take that measurement and times it by 3 and also by 5. This will give you your minimum and maximum size HDTV

Example:

The distance between where you want to place your HDTV and your chair is 9 feet. Take 9×3=27 so your minimum size would be a 27 inch TV. Now take 9×5=45, so your maximum size would be a 45 inch HDTV

Of course the other rule, which you should ignore, is that you buy the biggest one you can afford.