i3 doesn't let you define conditionals for per-host settings. So say you want to make your font larger only on your machine that has a 4k display (using 1 .i3/config) that becomes difficult.. However, as you may or may not know, you can define per-host conditionals in your .Xresources file using an #ifdef. Further, as it turns out; i3 now supports reading Xresources properties. As such, sharing an i3 config between hosts applying slightly different variables on each host for specific items is now trivial.

In your .Xresources put something like:

urxvt.font: xft:Monospace:pixelsize=10
#ifdef SRVR_somehostname
  urxvt.font: xft:Monospace:pixelsize=20
  *i3font: pango:Monospace 20
#endif

and then in your .i3/config, use i3's new set_from_resource command like so:

set_from_resource $i3font i3wm.i3font pango:Monospace 10font $i3font
font $i3font

And that's it. By default on normal hosts, your font will be size 10; and on your somehostname host, your font size will be 20 in both urxvt and i3. Rinse and repeat for other variables you might want to change.¹²