Be sure to measure your firing chamber before ordering to ensure proper fit, leave a minimum of 1/2″ on either side of the kiln for easy removal of shelves. Your kiln may be outfitted with 2.5″ or 3″ firebrick and that will determine the maximum size shelf you may use.
General kiln shelf sizes for standard shapes:
6 Sided kilns = 10″ shelves
7 Sided kilns = 13″ shelves
8 Sided kilns = 15-16″ shelves
10 Sided kilns = 20-21″ shelves
12 Sided kilns = 26″ shelves