I'm pretty sure my Pentagon I synth used to pop up in a properly sized window.
 
Now, though, everytime I add an instance of Pentagon, it comes up in a window that's too large causing a good amount of whitespace to the right and bottom of the window.  If I resize the window, then close it, when I reopen it, it's goes back to being the wrong size.
 
I probably screwed something up with all my learning/fiddling, but I can't figure out how to fix this.  Any ideas ?

I'm not sure that this is the case here.  Many of the older RGC synths would only display properly with a Display Properties / Advanced setting of 96 DPI.  The weirdness started with 120 DPI (large size) or custom settings.

Yeah, probably not the only bundled synth that can't handle custom display settings.  I would not be surprised if Cyclone and PSYNII worked the same way.

I guess you both are correct.  Pentagon, PsynII and Cyclone behave the same, everything else seems ok.  And this on both my laptop+big monitor and my desktop.
 
Though I do have a vague persistent memory of Pentagon 'becoming' wrong at some point, but I may have changed the screen resolution on one of my computers and not made the connection.
 
Thanks.
 

I had this problem when my setup included 2 screens of different sizes and resolutions.  As far as I remember, it occurred with all plugin GUI windows.