Google Webmaster Tools specifically call out both long and short meta descriptions as a problem, as well as duplicate meta descriptions. Duplicate Title tags are even more of an issue, as these weigh heavily with how Google calculates rankings. Most experts do agree that the descriptions meta is more important with how a page is listed than with how high.
Having said that, our experiences have been that ALL the supporting information on a page can be a factor in determining how relevant a page is for a given phrase or word, so make sure your Title and Description tags reflect the actual content on the page, and the more focused the message the better.
As far as how long the description should be, there is some controversy on the maximum size. 155 characters has been referenced for some time, but discussions on the Google forums seem to indicate a preference for 150 characters. You can make them longer; Google will just stop scanning at the cutoff. If you do, make sure that you have all the important info in the first 150 characters just in case.
Webmaster tools triggers the "short meta descriptions" notification when you drop below 50 characters, so I recommend having at least that many characters.