At 12~13' a 50" screen would be the smallest you would want to go. The rule of thumb varies from 1.5 to 2.5 depends on personal prefferences.
12'= 144", 2.5 times a 50" screen is 125". You could go larger than a 50" in your room if it won't break the budget. John
I am creating a media room & it looks like the main seating will be about 12 to 13 feet from the Plasma HD display. What is the rule of thumb about how big a screen is the optimum for different viewing distances?