Meta descriptions can be any length, but search engines generally truncate snippets longer than 160 characters. It is best to keep meta descriptions long enough that they’re sufficiently descriptive, but shorter than that 160-character limit.
Meta description tags, while not tied to search engine rankings, are extremely important in gaining user click-through from SERPs. These short paragraphs are a webmaster’s opportunity to “advertise” content to searchers, and searchers’ chance to decide whether the content is relevant and contains the information they’re seeking from their search query.
A page’s meta description should intelligently (read: in a natural, non-spammy way) employ the keywords that page is targeting, but also create a compelling description that a searcher will want to click. It should be directly relevant to the page it describes, and unique from the descriptions for other pages.
Not a Google ranking factor
Google announced in September of 2009 that neither meta descriptions nor meta keywords factor into Google’s ranking algorithms for web search. Google uses meta descriptions to return results when searchers use advanced search operators to match meta tag content, as well as to pull preview snippets on search result pages, but it’s important to note that meta descriptions do not directly influence Google’s ranking algorithms for normal web search.
That said, an effectively written meta description can entice people to click through to your site, and those clicks most certainly do influence rankings. For that reason among others, it’s important to put some effort into these descriptions.