There are two possibilities I know of to scale the UI...
1. Enable the debug console in AppOptions.txt (EnableDebugMenu 1).
In game, press ~ to activate the console. Use the command ui scale x , where x is a multiplier. So, fractional values <1 to shrink, and >1 to enlarge. You'll probably get best results if you use integers when enlarging.
2. Also in AppOptions.txt, there is a variable UIUpscale . I've not tried this yet, but it would seem to be a better option if you don't want to reset the scaling every session.