Meta descriptions can be any length, however up until now, Google generally truncated snippets to less than 250 characters. This limit has now been increased to 350! It's best to keep meta descriptions long enough that they're sufficiently descriptive, so I recommend descriptions between 50–300 characters. Keep in mind that the "optimal" length will vary. It depends on your business requirements. Your primary goal should always be to provide value and drive clicks.
Meta description tags, while not tied to search engine rankings, are extremely important in gaining user click-through from SERPs (Search Engine Results Pages). These short paragraphs are a webmaster's opportunity to "advertise" content to searchers. It gives searchers' a chance to decide whether the content is relevant and contains the information they're looking for from their search query.
A page's meta description should strategically (read: in a natural, active, non-spammy way) employ the keywords that page is targeting, but also create a compelling description that invites a web user to "want to click". It should be directly relevant to your message, and unique from the descriptions for other pages.