When my 42 inch Plasma arrived, I had to sit further away than normal, about 8 metres away to be exact!! the screen was just so big it hurt my eyes. I turned down the contrast, brightness, colour and sharpness which helped a bit. I took some paracetamol and got on with it. It was the sharpness that did the most damage I think
A few days later, I had no problem with the screen size increase (formerly only had 28 inch) and actually my headaches ceased and I started to turn up the settings. I now sit closer too and wish I had a bigger screen!
So I would say that you usually get used to the bigger screen after some time, and I would always get the biggest you can fit in without it looking silly in your room.
Beware of the pansonic PX70 though, I have one and although the freeview is great, the sound on it is only average (I have it through a Cyrus 8 Amp and Monitor Audio GS10 Standmounts), so you have to use satellite if you want a better sound. Also theres an annoying flicker on the screen in HD mode when the camera pans from left to right. Its the same on many TVs of this price, the refresh rate really goes down on panning, probably the processor is too slow.
Always try before you buy and check in the shop that it doesnt flicker on panning!
Graham