A Cre8asite Forums member asks the community about the length of a meta tag (keywords, description). Is there a particular length that you should limit yourself to?
Most search engines ignore it, and a recent lawsuit said that meta tags are immaterial. Still, a few people limit their meta tags to about 60 characters, like forum member EGOL.
Ammon Johns says something interesting regarding the length. His ideal meta tag size: zero. Why? You don't get credit and people steal it.
Seriously, there's a teeny advantage to not including the tag at all (leaner page, less data transfer), and a bigger disadvantage to having the tag in there (competitors mine it - half the keyword discovery tools use the keywords meta of other sites that rank well). Google doesn't give you any credit, not even a consolatory smile, for having the tag.
Kim Krause Berg doesn't use it and Barry Welford says that Yahoo will use meta keyword information for optimizing misspellings (but otherwise completely agrees with Ammon). There's more discussion as well, so hop on over to Cre8asite Forums for more.