Ok, after thinking on this one for a bit... let me quote John Palmer- " I often use a 15 gallon pot for mashing a five gallon batch."
He also reccomends using more water than some other homebrew gurus. So if you have Palmer's book (get it if you don't) and plan on following his guidelines on mashing, you will probably need to get a 10gallon cooler. I've been using a 10gal igloo (5-8 gallon batches) for a while and never had a problem with volume. I use Palmer's book extensively when writing a recipe.
For a typical Americal Ale that you want to end up around 5%ABV your grain bill will be somehwere around 12-15lb. So if you end up with a 12lb grain bill and use 1.5qt/lb, it might be hard to fit 12lb of grain and 4.5gallons of water in a 5gallon cooler.
Get a 10gallon cooler, or bigger. You can get a conversion kit and coupling from bargainfittings.com for any round 10gallon cooler (different one for igloo and rubbermaid, igloo has a thin wall so a weldless kettle ball valve works), and a bazooka screen from several online brewshops or maybe your local homebrew store. If you want to use a less expensive square cooler there are several online homebrew stores that sell full kits to convert them to mash tuns.