One of the gripes I've always had with the built-in Wordpress functionality is that you can only specify the number of posts per page across the entire application, as opposed to being able to customize it per section or page. Thanks to a wonderful plugin by Matt Read, you can fix this easily.
First, you'll want to grab the plugin from his website and install it. If you are unaware, that means unzipping and uploading the file into your BLOGROOT/wp-content/plugins/ directory.
Update: Looks like the original page is down, so here's a mirror copy:
He's got a donate button. If this article helps you, then donate money to him, not me.
Once the plugin is installed, you will navigate in your control panel to the Plugins page, and Activate the CQS plugin. Head over to Options / CQS and you should be presented with this screen.
I've already customized mine... you should change the Query dropdown to "is_category", set the Show condition to -1 (for all posts), and click the Add button.
I've also gone further and added a new condition so the Home page only shows 5 posts per page. I dislike cluttered homepages.