How big should a UIBarButtonItem image be?

Question

I'm looking to create my own custom Sort By Date and Sort By Number buttons that I plan on placing in the navigation bar as the right button.

How big should my image be to appropriately fill the space -- the UIBarItem documentation page doesn't list anything about the size the image should be.

Answer 1

About 20x20px, according to the Human Interface Guidelines. (A document that should definitely be in your bookmarks if you're working on iPhone apps.)