I’m not sure what you are asking. The default number of words for the excerpt is 120. To change it means really getting in there and playing with the source code. To control the actual size of the excerpt, in the Quick Tags above the EDIT/WRITE window where you type your post, there is a “more” button. Place your cursor where you want your “excerpt” to end and then click the “more” button. <!--more--> will appear in the text. This tells WordPress where to end your excerpt.
If you want a specific excerpt, then type it in the EXCERPT window in the EDIT/WRITE panel.
The 120 word limit is when there is not EXCERPT and the “more” wasn’t used. WordPress sets this as the default to grab the first 120 words of the post as the excerpt.
The Excerpt Template Tag will help you with more specifics, including changing the text for the “continue reading” link.
