Just a little information for other skinners who haven't figured this out yet:
If you create a SkinDefinition.xml file specifying a non-existant image file the client will load a grid pattern; allowing you to use the above method for getting the size without the need for creating an image file.
You can see the results of you image changes without reloading the client;
1. Load up the client and login to the game.
2. Load up your skin, this is how it is now.
3. Change back to the default & alt+tab into your image editor.
4. Make your changes & save them
5. alt+tab back in to the game & change back to your skin, you should see the update image.
P.S. the bottom toolbar is 1074 x 85 pixels.
I think something has changed, cause this does not work for me. It appears that I have to shutdown and start the client in order to get it to recongnize any changes in the XML file. Is it possible there is something I am not doing right?