So you
can just change the buff window(it's the EQUI_BuffWindow.xml file), but I haven't found it an intuitive process.
I modified mine to sit on the left side of the screen. Buttons on the left with text on the right. Buffs get a blue background, debuffs get a red background.
To address your question I played with the font size a bit. First buff is using font size 3, second is using size 5 and third is using the default, size 1.
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Looks like you can get to font size 3 without having to increase the size on everything, any larger and you'll have to start playing with the height of a bunch of objects.
You can grab the modified buff window xml here:
https://www34.zippyshare.com/v/057JJeFJ/file.html
To adjust font sizes, look for the buff label elements, starting with BW_Buff0_Label (I think there are 25 total, though we only use the first 15).
They will have a font attribute which you can set from 1-8.
The first two have already been adjusted to 3 and 5 respectively.