The length of meta descriptions, like the length of title tags, is subject to some rules and best practices based upon how search engines treat them. The first and most important rule is that search engines will only display a limited number of characters when displaying meta descriptions in search engine result pages. Google displays up to roughly 156 characters, including spaces, of a meta description tag. Bing will display up to 185 characters (in rare cases only, it's better to employ Google's standard of 156 characters).

Here's a wrinkle: Google will generally display your meta tag in search result pages, but not always. Sometimes, Google takes it upon itself to generate a custom description for display in search results.

Manipulating these results reliably is difficult, and probably isn't that worthwhile anyway. The most reliable rule; however, for how Google generates these custom descriptions is the following: if the search query is largely composed...