I agree, that you need the largest size that is practical.
Physical restraints come into play.
My first HD TV was a 50" plasma, bought in 2006. It has done well, and in 2006 the cost of a plasma larger than that was out of sight. At a 9' to 10' viewing distance it was too small.
The biggest problem was discongruity between the size of the image and the sound stage. The sound stage is huge, and for opera and some movies this issue was a problem and definitely detracted from the enjoyment.
So I recently put in a 65" screen.
This has placed the margins of the screen in the peripheral vision, at least for the front row. This has greatly enhanced the audio reality. The other bonus is that the increased size has made control of the HTPC much easier.
Now there are practical considerations. The screen would be better a little bigger. However, that is the largest practical screen size. The center speaker has been moved up as high as it can go. If it were higher, in any event, the center speaker driver, just above the screen would be too close to the ceiling, and if below the screen would be too close to the floor and not give optimal room coverage.
As in all things the results are the sum of the best compromises. In any event the change has definitely enhanced the AV experience.