The galleryWidth and galleryHeight entries in the 'config.xml' file are purely for JuiceboxBuilder-Pro so that, when you open an existing gallery in JuiceboxBuilder-Pro to edit it, the application can read the width and height values from the 'config.xml' file and populate the galleryWidth and galleryHeight fields in the interface (so that the same values will be re-used when the gallery is re-saved on the 'Publish' tab).

When the gallery is displayed in a browser, only the galleryWidth and galleryHeight entries in the embedding code are used so, if you really wanted to (and do not plan to edit your gallery with JuiceboxBuilder-Pro in the future), you could safely remove the galleryWidth and galleryHeight entries from the 'config.xml' file.