You can directly modify the .customui XML to change the labels, icons, and button sizes. However, if you are going to do that, you may want to bite the bullet and learn to do some XML modification in separate templates. Those can be independent of version and immune to MS updates. I have a number of modifications that work well in Word 2007, 2010 and 2013. The first one is a bit of a bump, but after that it becomes fairly easy. The links I gave you earlier are a good start. The last is a book. Although it is about Office 2007 ribbons, it is still the best overall guide I know.

They are stored in templates that are loaded as Global Templates. Take a look at my downloads page for a couple of samples that have Ribbon modifications. You can look at the XML in those to get a feel for it. They are the Gender Add-In, and my version of Chris Woodman's Keyboard Shortcuts Organizer. Virtually all ribbon-specific Add-Ins or tools on that page have QAT modifications.

Greg Maxey has pages on changing icons.