If you're on Wayland, use Sway instead. It feels so much like i3 that I often forget it's not i3. Hidpi works pretty well: https://github.com/SirCmpwn/sway/issues/797. I use this on a Dell Precision with the 4k display.

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.

What do you mean by "scale i3"? Just the text drawn by i3 or also the managed windows?