OK you said you’re smoke chamber is 14” dia x 24” length, which is 3,695 cu. in.
All components are based on CC size, there are formulas that work really well.
I suggest using .02 or 20 % where Dave uses .022 or 22%
.02 = 73.9 cu. in which is 23.52 in. for 2” pipe and 10.45 inches for 3” ( I rounded up to 12)
.022 = 81.3 cu. in which is 25.87 in. long for 2” pipe and 11.5 in. for 3”
Now this is a small smoker and I have not tested all the different sizes and lengths for this size.
Looks is also important, you don’t want a smoker with a ridiculously long stack as well as one that looks way too short. So look at your smoker and see what will look good, then size your stack. I don’t know if a 12” stack would look right or not, you could always go with a 16” and see how it works, if not you could cut it down. But a 12 or 14 may work.