I'm working on my external (non-Retina) monitor, with my Retina MacBook as a secondary screen next to it. Not uncommon, I believe.
When I open a document Photoshop displays it at Retina-size (i.e 50%) for a split-second before zooming in to 100%. It keeps the window size at 50% though. Since this can get a little annoying Photoshop should adjust the window size to show the full document instead.
(This happens with "Zoom Resizes Window" turned On and Off, as far as I can tell.)