Use xrandr --dpi 192 (or whichever value you’d like to use) before starting i3, i.e. typically in your ~/.Xsession.

i3 ≥ v4.13 will pick up this value from the Xft.dpi resource in ~/.Xresources as well, which is the more common way of configuring DPI.

edit: haven’t tested this within Xwayland, though. Note that i3 is only supported on X11.