Flue size

[thebomer]

I am building a 32" oven with a 16" height. The opening to the dome is 16" wide , 8.5" high on the ends and 11.5" high in the center. I am thinking about making my life easy for mounting the flue by getting an 8.5x18 flue and sitting it on the walls of the landing vs. building a smaller arch over the walls to hold a smaller flue.

Would this flue be too big?