I 'm either stupid or the documentation needs updating. Please let me know :)
On this page https://www.facebook.com/note.php?note_id=10150149060995844 says:
Change your "IFrame Size" to "Auto-resize" On your application developer page, in the "Edit settings" section of your app, in the "Facebook Integration" section, make sure that "IFrame Size" is set to "Auto-resize", NOT "Show scrollbars."
I can't find it on the developers site! :( Where on earth did they hide it? :o
The link you provided is not official Facebook documentation.
For information on how to build a Canvas app (including a section on sizing information), please see: https://developers.facebook.com/docs/appsonfacebook/tutorial/
This document provides information on how to make a fluid-sized Canvas app: https://developers.facebook.com/blog/post/549/