This is about the GM, so 'final' version of El Capitan on the Mac:
All Adobe CC/CC2014/CC2015 apps see this behaviour (note that CS6 apps are fine!): The size of the Open (and Place) dialogs are NOT remembered after resizing them.
Everytime you invoke a Open (read) command like Open, Place etc. the size of the Open dialog reverts to a small size. The output (write) dialogs are fine (save, save as, export).
This happens in InDesign, Photoshop, Premiere, Acrobat etc. since the CC versions.
Maybe Adobe is using a out dated call for NSNavPanelExpandedSizeForOpenMode?
Anyway: we tried a completely fresh install on a blank HD with El Capitan GM, then installed InDesign and Photoshop (so nothing was added to a fresh, new, 'blank' system) and the size was not remembered.
Now this setup has a multi display (three displays) on a iMac 27", but as it happens on a fresh new, further blank system install, I guess it is the call problem (as was the Big Grey box in Acrobat DC that got never resolved...)
If this is a CC error I would expect a bugfix in a release after start of El Capitán.
Yes, still not fixed, 6 months at least now.
I am beating the bushes like crazy looking for a fix.