Well, sadly it's age for me at least. I remember when my eyes were actually good enough. Now I have to use reading glasses sometimes to even see aspects of my UI. Basically we need /plugin mq2uifontsize5 loadthis comes up more and more. the easiest solution is to play at a smaller resolution.
short of that, like Sic said, you would need custom UI pieces. you can get these at eqinterface website. there was a guy making UI pieces exactly the same as normal UI pieces, but scaled the ui for a 4k monitor, searching for everquest 4k interface will net some results. you can also modify the files themselves. XML isnt really hard to work with, and you can increase the size of everything including icons however you want.
if you do run a custom interface you will have to periodically modify the UI pieces yourself or find others that work, and remake your Custom UI folder sometimes when eq modifies any of the default UI pieces.
Fun fact, brainiac has done a good bit of work recently to fix the mq implementation of the EQ UI so that it can be edited and loaded on the fly. Lots of great stuff there, would be worth it to explore of scaling UI is possible, though it might be obtuse.