IMO, within reason, it makes little difference. Obviously, you would not want a kettle with a very large diameter where it more resembles a shallow pan than a ketttle, but I would say that something with a diameter similar to the size of the grate on your burner would be about right. I would guess that large kettles are designed with the burner size in mind, meaning that if a kettle was too wide it would take up too much horizontal space. OTOH, if it was too tall, it would be difficult to access for stirring or visual inspection. Most kettles are used on some type of stove which would be higher than our typical propane burners at only a foot or so off the floor. The turkey fryer kettles are designed to accommodate a turkey while minimizing the amount of oil required, so they tend to be tall and narrower than a typical stock pot. If you were reducing maple sap, you would want wide shallow pans, but if you are making a stew, there would be no advantage to a shallow kettle that I can see.
