I'm assuming most UI's available out there were made by people in their 20's with 20/20 vision, heh. Probably not a lot of big-font UI's but you could look around.
Ultimately, you may want to familiarize yourself with enough SIDL to make this change yourself; on the bright side, it may not be that bad when it's all said and done.
Trying to mentally parse the entire contents of an EQ UI all at once is too overwhelming, IMO; there's quite the mountain of crap you can customize in there. All you really care about are font sizes, so that lets you ignore tons of irrelevant stuff. Given the nature of XML, it may also be possible to leverage just a few strategically placed changes in order to propagate a font size change across the majority of the UI. And if that's the case the task may not be so daunting as it seems at first.
There are likely quite a few elements throughout the UI definition that display text and therefore have a font size. However, if all such elements inherit from a base element (like if for example "STR_Label" and "STA_Label" both inherit from just plain "Label"), rather than editing each and every single different label throughout the entire UI, perhaps editing the base "Label" from which they all derive could be a shortcut to your goal. If this is the case, the only real work to do would be locating the correct base elements for all text-rendering types in the UI (such as "Label", or whatever else). You may be able to hack together a big-font UI without actually having to change very much data... if it's structured this way.
I'm speculating based on my knowledge of similar systems. Short of getting some pointers from someone who's actually authored an EQ UI, you'd have to snoop around in there, find out what you're up against; EQ installs SIDL documentation at {your eq folder}\uifiles\default\AboutSIDL.doc