HT room will be 18 ft wide and 23 ft long. Want to go with 16 X 9 screen. What would be a good size screen.

Just based on the info from the new S&V disc i watched yesterday, i think they recommended you place your viewing position about 3 times the distance away from your screen that the screen is in diagonal width.
e.g. 50" screen means you sit about 150" away (about 12.5 feet)
Given that approximate information, you could decide about how far away you want to sit from the screen (23' is a pretty long room) then back calculate an estimate of the screen size.
Personally for me its going to come down to budget more than anything but if i end up sitting about 8 feet away from a 50" hdtv, i'm just going to love it regardless of how warped my eyes will become.
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hometheatreforum.com is another good place to look for info on home theatre setups esp. in regards to big screen tv