Process: Adobe Photoshop Lightroom 5 
Path: /Applications/Adobe Photoshop Lightroom 5.app/Contents/MacOS/Adobe Photoshop Lightroom 5
Version: Adobe Photoshop Lightroom 5.7.1  (5.7.1)
Code Type: X86-64 (Native)
Parent Process: ??? 
Responsible: Adobe Photoshop Lightroom 5 
User ID: 501
Date/Time: 2015-12-17 14:20:01.391 -0500
OS Version: Mac OS X 10.10.5 (14F27)
Report Version: 11
Anonymous UUID: 55E9DF3A-D5A7-C1D7-DB5A-018186370DEA
Time Awake Since Boot: 1700 seconds
Crashed Thread: 33
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x000000000000001
I doubt there is a solution for Lightroom 5.
If you upgrade to Lightroom 6.3/Lightroom CC 2015.3 (which means you have to spend money), and you have a powerful enough graphics card, you should be able to take advantage of the entire resolution of your Retina screen.
Thanks for the encouraging comments. I would gladly update to LR 6.3 if I could be certain it will solve the problem.
Before laying out the money though I would appreciate hearing from any iMac Retina users who have successful experience with LR6 running slideshows in full screen full resolution.