I bought my 42" TV nearly 4 years ago. It was at a price point where a 60" TV sits today; slightly below the groundbreaking 65" screen. My wife wants to go for a bigger screen now but I am not convinced it is a good idea. People are still buying into the idea "bigger is better", just like camera buyers are led into thinking more megapixels means better quality.
HD (high definition) TV broadcast ranges in resolution from 852x576 up to1920x1080 pixels. It doesn't get better than that unless somebody in the broadcast industry decides that it is now time for a change. And change is expensive. Blu-Ray resolution is 1920x1080 while DVD resolution is normally 720x576 (PAL) or 720x480 (NTSC). So the maximum resolution is about the same whether you are watching the most advanced digital TV or playing from the most advanced media player.
The question should be how big do you need a screen to be to enjoy watching 1920x1080 pixels. Try watching a low resolution image, say 320x240, on a big screen and you'll quickly notice how un-watchable it is. The point is, there is a comfortable screen size/viewing distance for any given resolution. I won't pretend to know what that is but there is a limit to how far you want to go. Generally for a given resolution a big screen is good for a big room to accommodate more people. For a small room, a giant screen simply makes every pixel larger and you will not enjoy high definition TV. All you get is a big pixel TV unless your room is large enough for you to sit far away.
Oh, I do know the optimum screen size for my TV room. It is 42".