To calculate the necessary image size, measure the largest viewing distance (the distance between the image and the furthest chair in the room) in inches and divide that number by eight. The resulting number is the display height necessary for effective viewing.
For example, a viewing distance of 240 inches (20 feet) requires a 30" image height, which is approximately what is provided by a 60" flat panel.
Many designers recommend even larger image sizes when small text or detailed graphics are used. In this case, replace the above image height factor of eight with either six or seven.