You want to be staying under 160 characters for Meta Description. If you are using wordpress you can use Platinum SEO pack and it will count the characters as you type so you know how many you have left.
Although there is no real limit, the <title> is ~64 to 70 char long. Desc is ~128 char long. Keyword is 64 char long. But the length is not something you need to worry about. Just look at the use of the keyword phrase from the top 10 SERPs (view source) and keep it under or even on what then did. Then start writing orginal and good SEOed content.