SEO

Description meta tag

For a discussion of maximum length and content see Dan Hounshell's blog and the Google Webmaster blogs:Improve snippets with a meta description makeover and Changing your site's title and description in search results

In conclusion the first 150 charactes will be shown to the user so write for a human. Google will use more characters for the index if the algorithm finds them appropriate so you might want to go up to 250 characters or even more for machine processing.