It isn't accurate to say that any particular diameter will flow better than any other diameter.
Airflow in any particular pit is a symbiotic function of firebox size and layout, fire grate placement, vent size and placement, cook chamber shape, exhaust stack design and placement, the quality of the door seals, rack placement, the shape and size of air current passageways, fire and firewood management; and often, a pinch of magic.
Or, just follow the well beaten path to the pit calculator at the head of this forum and avoid all the pole vaulting over mouse turds that we all seem to be guilty of occasionally...... maybe me more than most......
Suicycle
, if you are going to be in anything up to about the 24" range you could do a lot worse than to cruise
Amazon for some of the better gas
grill grates. You can buy some pretty heavy duty ones made of a good grade of stainless steel for less than buying the material. Then you can decide whether to use them as-is as drop in grates, to build a sliding frame for a movable rack, etc.
I cut one end off of a 20-1/2" OD propane tank a few months ago for a smallish trailer mounted reverse flow smoker with a warming/cooking cabinet above the firebox and sized the layout of the cook chamber and warming cabinet around commercially available gas
grill grates. When it cools off a bit here in North Carolina I'll get some pictures and get on with the build.
Here's an example of off-the-shelf grates that might serve well in a smoker of roughly 20x40":
And some for a 24x40 smoker:
Best regards to all,