Scenario:
"I only want my RSS feed to show the first few paragraphs of my items because I want people to come to my site to read the full item. How can I do this?"
Solution:
Go to your Nucleus admin area and click on "Templates".

Click on the "Edit" link next to the template "feeds/rss20".

Find this code in the Item Body section of the template:
<description><![CDATA[<%body%><%more%>]]></description>

<description><![CDATA[<%body%>]]></description>
Original forum thread:
http://forum.nucleuscms.org/viewtopic.php?t=11637