I was going to work on this today but you already did some of the xml code
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Points to remember:
1. Hotbutton and cast spell window gems are always the same size. The icon info is pulled from the spellgems code in the Animations.xml.
2. The spellgem locations can be offset in each xml window with the SpellIconOffset tag.
3. The backgrounds would need to be redrawn to match their new size and their code changed in the castspellwnd.xml file.
Basically, although its possible to do I would just stick with the standard size (unless you want to redraw a ton of files).