[taa]

As a general rule, shorter URLs are more memorable, better for SEO, and better for social sharing.

Memorable: 7 letters or less is best, 9 is still manageable, and anything more than 9 calls for misspelling, too long to type, etc. I personally believe that a 2-word 12 characters name is better than a 5 letter weirdo name or random characters, but I haven't seen a study to support that.

SEO: the URL shown in google SERPs plays in a role in clickthrough rate. A shorter domain name means more of the URL will be shown, as many services limits the maximum number of characters shown.

Social sharing: same as above, short and meaningful domain names are more likely to get clicks and be shared again than a URL that passed through bit.ly and friends. Doesn't apply as much to twitter, but think email, facebook, linkedin.