The size of the smoke stack and fire box all depends on the size of your cooker. Round works just fine as long as it is big enough. There are a few smoker size calculators online where you can enter you size data and it will tell you how big the fire box and smoke stack should be.
Matts right on that the shape of the stack isn't a giant factor. The sizes are what count.
For the.fire box, I have cooked on round and square. The smokers were two different animals, so its hard to say which.is better. It's a lot like the stack in the sence that the flow has to be right. I'm sure we can get ya fixed up tho.
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I prefer a square firebox over round, inch for inch the volume is just there with square fireboxes. I have used round and will again but mostly because of cost pipe is just less expensive than plate. And size does matter where fireboxes are concerned I prefer larger fireboxes.