Choose the size of a ceiling medallion based on the room size, the height of the ceiling and the ceiling fixture size. A tape measure is needed to determine the size of the room.
Measure the room
SFGate suggests starting by measuring the length and width of the room. Multiply the two values and divide the result by seven to obtain an estimate for the appropriate size of the ceiling medallion in inches.
Compare your calculation with recommendations
This Old House suggests using a medallion up to 20 inches in diameter for rooms that are 140 square feet or less. Use a 20- to 30-inch medallion for rooms up to 210 square feet, a 30- to 40-inch medallion for up to 280-square-foot rooms and a medallion of 40 inches or larger for rooms of 280 square feet or more.
Check the height of the ceiling
When the ceiling of a room is high, it gives a more focal outlook. According to SFGate, higher ceilings can accommodate larger medallions, while smaller ornaments are more suitable for lower ceilings.
Note the size of the ceiling fan
The size of the medallion should also fit the size of the ceiling fixture. According to SFGate, medallions should be either larger or smaller in diameter than the light fixture. If the medallion is installed above a ceiling fan, the medallion should also be smaller in diameter than the fan.