I see from the mod queue that you figured it out. I was gonna suggest taking someone else's skin apart....
I've used 16 pixels to 23 pixels high.....I usually leave the top & Bottom pixel rows magic pink and add magic pink space at both ends.
Length is only what you need to get everything to show based on your margins
edit: you can't use anti-aliasing on your corners...you end up with pink that's not quite "magic"