Ctrl=, Ctrl-- or Ctrl-mousewheel
In a fit of pique this evening, I added a much requested feature to the NetBeans output window: Actions for Larger Font and Smaller Font on the popup menu - or you can just Ctrl-mousewheel to rapidly change the font size.
Font changes affect all open output windows, and persist across sessions.
It just seemed silly to have this issue sitting open for so long. Now we'll see if anyone gets upset with me, since I haven't actually been the owner of the output window code in years. :-)