chains vary in width, but 5s and 6s chains are the same. 7s and 8s chains are a bit narrower but will still work fine. 9s and more will not only cost you more, but will be a bit too narrow causing problems like getting caught between the chainrings,

As JMC said, your chain will be too long in the box and you'll need a chain too to cut it to length. If you buy a 5s or 6s chain (if you can find one) odds are it'll be of a construction where you can both cut and splice it with the chain tool. These chains are recognized by pins that protrude about 1/32" beyond the plate surface. 7s and more chains will probably have pins that are almost flush with the outer plates, and must be spliced with a connecting link or special pin.