Google will choose your search results snippets from the following places (not necessarily in this order):
- The page's Meta Description tag
- The page's Open Directory Project (ODP) Listing
- Page content relevant to the search query
If you do not want Google to use the ODP listing's description then you can tell them not to do so with the following Meta tag:
<meta name="robots" content="NOODP">
If you want to encourage Google to use your Meta Description tag then make sure it is unique to each page. Also make sure it contains an accurate description of the page's content.
In thew absence of an ODP description and Meta Description tag, Google will use a portion of the page's text as the description. This text will contain the closest matches to the search query. I have not seen any official limit to how long this can be but a couple of sentences seems about right.
On a related note, if you don't want a snippet to be shown with a particular page you can use the following Meta tag to prevent one from being shown:
<meta name="robots" content="nosnippet">
See this blog post for Google's tips on using the meta description tag.
According to this site, "The meta description should typically be at most 145 to 150 characters in length as these are the maximum number of characters typically displayed at Yahoo! and Google, respectively."