I'm using a Mac Pro early 2014 (dustbin) with 2x 27 inch monitors. All my Adobe apps are having the same problem. If I resize the Open/Save As dialogue box they are always defaulting back to their default small size. This didn't happen with CC2014. All Apple apps are OK plus other 3rd party apps are fine too.
What can I do?
That means you have El Capitan (OS X 10.11) installed. All Adobe programs (InDesign, Photoshop, Illustrator, Premiere etc etc) have this problem with read-dialogs (Open, Place) but not with write-dialogs (Save, Save as, Export).
It only happens with El Capitan, all CC versions but not with CS6.
Adobe is aware and they should fix it.
Thank you Frans. I thought I was going mad!
Mike, can you hit the 'Correct answer' button under your message? thanks!