The architectural correct way to pick the size of a medallion:

Multiply the length of the room; then divide by the number of fixtures in a room ((L x W) / # of fixtures) = X

Using the chart, pick the appropriate ceiling height. Divide the number above by the fixed divider.

 

Height of Ceiling	Fixed Divider
8' - 10'	10
12' - 14'	8
16' - 18'	6

For example: Length = 20';
Width = 25'; # of fixtures = 2;
Ceiling heigth is 12'; then:

20'(L) x 25'(W) = 500' /2(# of fixtures) = 500 / 8(fixed divider) = 31.25"

Your would need two medallions that are approximately 31"W.