Make the Post Content automatically the Excerpt AND the <description> in RSS

My goal is to just type in text in the Post and have the Exerpt automatically be the same, at least in my <description> field in the RSS feed. The reason is that <description> doesn’t have encoding, and that’s what I want. But it also cuts off at 120 words. Does anyone know how to make the <description> either limitless or increase the default?

I’ve seen some forum posts about how to do it, but it’s just confused me, and I’ve experimented with my wp-rss2.php file, but nothing has worked.

I’m using 1.5.1.2.