I have a Masterbuilt 44 inch propane smoker. The exhaust vent in the top of the back of the cabinet consists of four 1/2" X 2, 1/2" slots. The slide is designed to block off two slots when closed, reducing the vent area by half.
The total area of the opening equates to a circle of 2,1/2" in diameter. Is that about right? Seems to me the exhaust vent should allow for more venting if desired, depending on what's being done.