Using the chars attribute tells the template engine to only show that number of characters when rendering the field:

<perch:content type="text" id="heading" label="Heading" chars="100" />

The words attribute has the same effect, this time restricting the number of words:

<perch:content type="text" id="heading" label="Heading" words="20" />

This can be very useful if you are creating a list/detail page setup using perch_content_custom() as described in this solution.

If you wanted to show an excerpt of your article on the listing page, limited to 30 words, you could use the following:

<perch:content id="body" type="textarea" label="Body" 
    required="true" words="30" />

There is information about words and chars in the Perch documentation.