The default excerpt length in wordpress is 55 words. It is a pretty good length I think. You can adjust the length by editing your functions.php file in your template folder see the codex for more info.
However sometimes you get a situation when building a template where you need a smaller or larger length of excerpt somewhere in your theme. The way I use is quick and easy, but it does the length by character not words and it will generally strip your paragraph tags which is why I have wrapped it in them.
That will pull an excerpt of 200 chars, adjust as necessary.