Encryption is done with a key, which is a set of binary bits, not a password, which implies a human-readable string.
To go from a password to a key, one can use a Password Based Key Derivation Function, such as PBKDF2. Crypto-JS already has a PBKDF2 function built-in, i.e.
<script src="http://crypto-js.googlecode.com/svn/tags/3.1.2/build/rollups/pbkdf2.js"></script>
<script>
var salt = CryptoJS.lib.WordArray.random(128/8);
var key128Bits = CryptoJS.PBKDF2("Secret Passphrase", salt, { keySize: 128/32 });
var key256Bits = CryptoJS.PBKDF2("Secret Passphrase", salt, { keySize: 256/32 });
var key512Bits = CryptoJS.PBKDF2("Secret Passphrase", salt, { keySize: 512/32 });
var key512Bits1000Iterations = CryptoJS.PBKDF2("Secret Passphrase", salt, { keySize: 512/32, iterations: 1000 });
</script>
In general, use as high an iteration count as you can get away with.
Salt should be a random value, as in the example above; you'll need, of course, to store that value along with the iteration count in order to get the same key given the same passphrase.