I spent a bunch of time today trying to get a mx:Accordion to respect my width="100%" and height="100%" parameters. Little did I know that I needed to set the resizeToContent attribute to 'true' in order for the Accordion to update its sizing when content is added to it.
Here's what the definition would look like this:
<mx:Accordion width="100%" height="100%" resizeToContent="true">
I hope this saves someone just a little bit of time when trying to get their Accordion to resize properly.