5-gal is plenty.. .a 3-gal is good for all but the "XL" autos that exist.
whether in soil or coco may matter.. but 5gal with auto should be more thnan ample in soil too...
i usually check rootball after i harvest to see if it's crowded or other issues... there's plenty of space in a 3-gal after a grow with a ~normal autoflower.