What is the correct way to handle HiDPI on X11 (SDL2)

I know people are moving to wayland, but some people are still stuck on x. How do I make a window render using correct proportions on X11?

I have just created a language binding to Nuklear and the demo application renders very small. It has the following line:

   Hints.Set (Hints.Video_High_DPI_Disabled, "0");  -- SDL_VIDEO_HIGHDPI_DISABLED