Diameter doesn't matter. (In theory.) However, if the inlet area is very small and the attached plumbing has a very large volume, you may have a sluggish response to change in speed. Worry more about location, proper aiming, weight, and cost. But do use a size that you can clean out with common tools ("pipe cleaner", "toothpick", "coat hanger wire") if you get a bug or other garbage in the pipe. I'd go for maybe 2 mm or 1/8 inch.