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There is not one overriding value for the default fonts, each UI element tends to have its own.
First of all, grab one of the user created UI mods and you should see a Fonts folder. Open this folder and open up the fonts.xml file.
This will have all of the additional fonts defined (as well as sizes for said fonts). Normal fonts you can find throughout the XML code in the Source folder.
Each font will be defined by a name that refers to the definition in the fonts.xml file (i.e. in my Third Dawn add-on, UO_15 is the classic UO font in 15 point, magic_20 is the Magic the Gathering font in 20 point and so on).
In the XML files in the source folder, you will be looking for something to the effect of <font="font_xx">. That is where the font is defined for each element within the element itself.
You will then find that many elements, especially ones that are used in various UI windows will be referenced. Most of those will be found in DefaultUITemplates, DefaultWindow, and so on.
By messing around with the XML code you can change the fonts and sizes of the various elements to your liking (if it is not doable in the skin settings itself).