In ~/.Xresources (example):
! Xft.dpi: 96
Xft.dpi: 192
.
.
.
!URxvt.font: 9x15,xft:TerminessTTFNerdFontMono
URxvt.font: xft:TerminessTTFNerdFontMono:size=14
That's all folks!
This controls the dots per inch used by most X11 apps, and also the font size in URxvt
(the default terminal).