Earlier today I discovered by accident that you can press Shift+PGUP or Shift+PGDN inside st(1) to increase or decrease the size of the text.
Wha?? Why isn't this awesome feature documented anywhere?? Furthermore, I confirmed that xterm(1) and urxvt(1) don't have that functionality. At least with that particular combination of keys.
Anyway, that's pretty frackin' awesome — you know, PFA.