Chains come "out of the box" to fit anybody's bike -- i.e., there are more than enough links on a standard chain to fit your gearing setup. For optimal shifting though, you need to size the chain for your gearing.

There are multiple ways of determining optimal chain length. I've always gone with "wrap the chain around the big chainring, the big rear gear, draw it tight, and add two links" (bypass the rear derailleur if you do this).