In preferences -> shortcuts, there is no possibility to define a shortcut to increase/decrease font size for the active terminal window, which leads to the problem that the only way to alter the font size is via Appearance -> Font, which in turn adjusts the font size on all other terminal windows open, see:

https://bugzilla.xfce.org/show_bug.cgi?id=7975

A default keybinding using Ctrl-+ or Ctrl-- (that is control and the "-" key) would come to mind.