proper size of vent opening

[paulages]

has anyone figured out a ratio for the diameter of the oven to the size of the vent opening?
i have a 42" floor, and my vent opening can be up to 18" wide. any suggestions for the depth of the opening? optimal with my design would be 12" x 6", but i'm not sure if this will draw well enough.

[ColonelCorn76]

12x6 will be fine -- that's about what mine is (I used a 13x9 clay flue which has rounded corners so the vent actually tapers into something more along the line of 12x6 before entering the flue as I didn't round the corners of the steel). It's about 3' tall.

An 8" diameter chimney is also a good size and matches pretty standard stove chimneys so they're readily available.

Jim