I'm using a 15 inches screen and uses the resolution 800x600, I don't use any higher resolution unless I absolutely have to when browsing the web.
This is is because I'm very near-sighted.
If you go in-game and press the ESC key, and then go to "Options", then "Advanced", there should be a UI Scale option there. Try adjusting your UI scale.
In addition to that, your chat window should have options to increase the font size as well. If you right click on one of the chat window tabs, and select "Font Size", you should be able to adjust it to your liking.
I knew about chaning the font size and change the chat windows background color.