As notas de lançamento para i3 4.8 cobrem isso:
Also very important for owners of HiDPI/“retina” displays is that i3 will now respect your configured DPI and scale up its UI elements accordingly. Use “xrandr --dpi 184” to set your dpi to 184, in case your setup does not figure it out automatically. To get properly scaling fonts, we also changed the default font from a bitmap font to a pango font (“DejaVu Sans Mono 8”).