It is unclear what purpose the pan would serve?
Are you trying to capture drippings? Although many posts frown upon using a pan, a loaf pan (or two end-to-end) placed on a rack below the meat should leave plenty of space left and right for heat and smoke. I would suggest the pan for drippings be no wider than the width of the smoke box.
If you are talking about smoking meat by sitting it directly in a pan, then the pan should be as small as possible, trying to leave as much space around it as possible. This is generally not recommended though, unless you are smoking sides, etc. It is much better to smoke meat directly on the racks, and not inside a pan.
If it is a water pan for moisture, you should place a loaf pan with liquid on the floor of the smoker snugged up right next to the smoke box.
