This might help.
Ultimate HDTV buying guide
According to this, assuming your looking at 16 x 9 HDTVs which will have the screen approximately 11 feet from your seating area (that's allowing for a set approximately 18" to 24" deep), sets from from 42" on up will be OK.
Keep in mind that the figures in the chart are for DVD and HDTV watching. If you'll be watching a fair amount of standard TV, it might be prudent to get a set that would put your seating area in the middle of it's viewing range. If you get a larger set, you might be sitting too close for Standard TV resolution.
For example a 60" set, with a seating range, according to the chart, of 7.5' minimum to 15' maximum would have your 11' distance approximately in the middle of that range.
If your watching will be primarily DVDs and HDTV, then get the biggest you can afford!!!!!!!!