25" x 14 3/4"   extends 15 1/2" 
21 1/2" x 12 3/4" extends 13 1/2"
18 3/4" x 11 1/4" extends 12" 
16 1/4" x 9 3/4" extends 10"  $
14" x 8 1/2"   extends 9 3/4" 
Vent caps and finials are design options.

Shown: 
With Spear Point Finial and no vent cap
18 3/4" x 11 1/4" extends 12" 
 

With custom grid pattern

Tapered version of the Cupola Lantern
18 3/4" x 11 1/4" extends 12" 

With vent cap and Spear Point Finial (top)
 

Without vent cap or finial

Vent Cap with short finial (bottom)