To calculate the fandelier size, add the room's width (in feet) times length (in feet). For rooms 12 feet wide and 16 feet long, you would need a 28" wide fixture. Another thing to consider is how tall you want your fandelier to be. You need at least 7 feet of clearance for any light fixture. If you have an 8-foot ceiling, you should get 12 inches or smaller fandelier.