In my humble opinion, a 5-gallon beverage cooler is the perfect size for normal gravity 5-gallon batches when continuous sparging, as it provides for a better bed depth than a 10-gallon beverage cooler. A 5-gallon beverage cooler also makes for a good 3.5-gallon normal and high gravity tun when continuous sparging. That being said, you will probably opt for batch sparging, which is will work with just about anything that will hold temperature and take some kind of straining device. With batch sparging, ease of stirring should be factored into the equation. It's easier to stir in a picnic cooler than it is a beverage cooler.