Room: 16 x 18.5 how big of tv?

Take your viewing distance and divide it by two, that should be your screen size.

10-11 ft will never be too big for 90 inches and below with any HD signal, DLP Projectors can give some showing of pixels as you get cclose but even then the distance of 10 or so feet is a non issue.
Get as big as you can and you might be shocked to find how cheap Projectors are if you can control light and dont need to cycle it on and off many times per day................there is nothing like a great flic or even a concert DVD on a nice big screen.sports too!

It isn't perfect or followed by any particular guidelines but here it goes:
I have two displays in my listening room.
* 50" 720p Pioneer plasma
* and 1080p Optoma DLP projector and 84" motorized/tensioned screen

Viewing distance is 12' to plasma display and 11' to motorized projection screen.

Pioneer is great on SD as well as HDTV and looks better then my newer 47" LCD 1080p display but nothing in comparison to "big screen experience".

Depends on your seating distance. I wouldn't go below 100" diagonal for an 11' viewing distance or 110" for 12'.

Seating at 1.5 x screen width for a 16:9 screen is an OK compromise which doesn't make lower quality sources too big or good scope DVDs too small