How to set Notepad++ GUI text to a bigger font?

The GUI is made of standard windows drawing primitives. That means standard controls apply to them. You can change the overall appearance of all windows menu fonts or you can use DPI scaling. Windows 10 Creators Update (1703) has a new DPI scaling mechanism that should help more with applications like Notepad++.

I was able to do it by changing the registry key...

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes\MS Shell Dlg

to something like...

Arial Unicode MS

The prior value of that key on my Windows 10 machine was "Microsoft Sans Serif", an older font that I had some problems with before in another app. So, although changing the font substitution (as shown above) is a system-wide setting that will affect all of the (legacy) apps on your system, in this case I didn't worry about it too much. Especially since, in my opinion, the situation with the Notepad++ UI font couldn't get much worse.