The Wash. Dept. of Wildlife Urban Wildlife Series pamphlet on Nest Boxes for
Birds gives the following info for the Barn Owl:

Floor of cavity - 10"x18"
Depth of cavity - 15"-18"
Entrance above floor - 0"-4"
Size of entrance - 6" dia.
Height above ground - 12'-18'

This pamphlet also give specifics for 16 other birds.

Hugh Jennings
Bellevue, WA