xfce4-terminal doesn't provide a facility for {in,de}creasing font size in a running terminal. However, xfce4-terminal does watch the file $XDG_CONFIG_PATH/xfce4/terminal/terminalrc for changes to the FontName directive (which includes size).

Therefore, this script can be bound to a via a keyboard shortcut that will change the font size, albeit after a short delay.

No thanks to urxvt-unicode for not supporting fontconfig, which is necessary to run an unpatched Monaco font in Powerline, which I must have, all of which prompted my switch to xfce4-terminal which does support fontconfig and ... oh, nevermind.