Hey, sorry for opening up an old thread, but I reckon this is the first one a newcomer stumbles upon searching for a solution to this problem. I know, I did.
Anyways, to fix this on windows 10, locate the spartan.exe in your gamefiles, go to properties->compatibility->change high DPI settings and change the value under "High DPI scaling override" from application to system.
It's by no means as good as a UI slider would be, but it makes the UI so much more readable on a big screen!