Smoke stack

[coho]

Need help with the size of the stack cc is 18inches x 45inches firebox is 18inches x 24inches. Throat is 13inches wide x 6 inches tall football shaped. I'm thinking 4 or inches for the stack but I'm not sure

[k.a.m.]

If not already built I would reduce the firebox length to 18" this would put you at about 20% over size. I would opt for a 4" ID exhaust 24" tall.
Link to BBQ Pit Calculator

[coho]

I appreciate your help. But I don't understand why 6 inches of firebox will have that effect. Is it to hard for it to draw air the extra length.

[k.a.m.]

I appreciate your help. But I don't understand why 6 inches of firebox will have that effect. Is it to hard for it to draw air the extra length.

I build cookers using the pit calculator. The program is designed around a 1/3 rule sizing the firebox to the cook chamber.
I ran your numbers and came up with this:
Link to BBQ Pit Calculator
The firebox was coming in at 60% larger than needed.
I reduced the size and came up with this:
Link to BBQ Pit Calculator
The firebox is about 20% larger which in my opinion creates a more balanced cooker.
Will the larger firebox work yes by all means and once you get used to running it you should be fine.
The smaller firebox in my opinion would be easier to maintain temps.

[coho]

That's making more sense to me now thank you for your help

[Weaverspitbbq]

Where you place your wood in the box also regulates your temp ...