I have a large screen, and managed to resize most parts of the GUI in settings. But remaining is the resize handles, which are very small on my screen. This Inkscape feature is quite unusable when they're so small. How do I resize them?

If you mean the arrows that appear around an object when you use the tool, for resizing, skewing and rotating, then I don't think it is possible to change their size. The handles that appear when you double-click on an object (e.g. the resize and corner-rounding handles when you use the tool) can be changed in the Inkscape Preferences on 0.92 (Edit > Preferences then Input/Output > Input Devices > Handle Size).

Thank you