Maximum Length of Meta Descriptions
It is becoming more and more evident that meta descriptions with more than 145 characters are getting truncated, particularly on higher ranking sites, which coincides with the limit on a Twitter "tweet". Meta descriptions still get used for some search snippets, although more and more snippets are using descriptions that the search engine is selecting on its own, so length considerations are important. Since the fact that length is a reflection of how much gets shown in a snippet, it only makes sense to also limit the length we put into the meta description.
Still, the fact that we can still control the meta description and gain some value for it make it a worthwhile thing to which we should pay attention. I don't know yet what the lower limit is, but based on what I see in Google Webmaster Tools, Google tends to warn about short meta descriptions, which tend to be descriptions that are 50 characters or less in length. So as a working hypothesis, it seems reasonable to make meta descriptions fit into the range of 50 to 145 characters in length.
Of course, as always we should remember to make the meta descriptions useful. Google's beginner SEO guide says, "Write a description that would both inform and interest users if they saw your description meta tag as a snippet in a search result practice in case Google cannot find a good selection of text to use in the snippet." The keywords here are "inform and interest". As with everything else we do on a web page, the meta description, first and foremost must say something useful from a user's perspective.
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