You can try using the THX viewing distance calculator.
Keep in mind, though, that some people feel that the THX guidelines result in a screen size that's too large. I certainly wouldn't exceed the screen size recommended in the THX guidelines.
Another thing to factor in is the resolution of the "average" software you will be viewing. Large screens with near viewing distances might look great in high def, but it will make the resolution limits of standard def more perceptible, often unacceptably so.
Good luck.
Bryon
Keep in mind, though, that some people feel that the THX guidelines result in a screen size that's too large. I certainly wouldn't exceed the screen size recommended in the THX guidelines.
Another thing to factor in is the resolution of the "average" software you will be viewing. Large screens with near viewing distances might look great in high def, but it will make the resolution limits of standard def more perceptible, often unacceptably so.
Good luck.
Bryon