This is a problem I have been having recently. My screen size has a pretty high pixel density and, as a result, all of GIMP’s icons are criminally small and very difficult to look at.
I’ve tried searching for ways to increase the screen size but all that shows up is how to scale images. Thing is I know how to change the image DPI but I want to know how to change the actual program’s DPI, so that the icons are better suited to my computer screen.
A simple solution for GIMP 2.10 (both under Linux and Windows):
Go to “Edit → Preferences”.
In the Preferences dialog, go to “Interface → Icon Theme”.
From the dropdown at the bottom, choose “Use icon size from the theme” or “Custom icon size” (and in the latter case, choose your icon size with the slider below that).
The problem you describe happens because by default it’s “Guess icon size from resolution”, which considers only the resolution such as
1920×1080 px and not the physical size resp. DPI settings of the display. Worse, in a multi-screen setup it uses the combined resolution, which is the resolution of a virtual screen of which each physical screens shows a certain part.