Last time I looked into it, google's wmt starts yapping if your meta description is less than 50 characters. Bing's cutoff is a bit less, maybe 30 or 35.
If you're thinking of the meta description as an updated form of
<meta name = "keywords" stuffing = "blahblah">
... don't bother. The meta description itself isn't indexed-- as least not by google-- but it's used instead of a snippet if a multi-word search doesn't fall conveniently into a single sentence or at least paragraph of your page. (I don't really know this. I just did some informal experimenting once.)
Search engines have also experimented with creating snippets of a certain width and not a specific character count. I would worry less about optimizing meta description and worry more about creating value that isn't available on 100 other websites.