It's a simple math calculation to get the optimum TV screen size for your viewing distance. The formula is:
Minimum TV dimensions = Viewing distance / 3
Maximum TV dimensions = Viewing distance / 1.5
From this formula, we can calculate that with 9 feet viewing distance, the beset TV dimension should be around 36-72 inch. However, it's still recommended to buy the TV size that is closer to the minimum TV dimension to guarantee your eyes' health.