I created a template for our book covers in InDesign CS5, with separate master pages for front, back and spine. I set the file up so that I could adjust the width of the spine page quickly and easily using the page tool and entering a new width on the measurements toolbar.
I upgraded to InDesign CS6 a few months ago and cannot get the spine to adjust in the same way as before. The page size changes but the frames, rules and text boxes don't adjust dynamically as they did in CS5.
The attached images show what happes in CS5 before adjustment and after, as well as CS6 before adjustment and after.
I have played around with various settings, but cannot get it to work in the same way.
I think that your objects adjusted in CS5 when you changed page sizes because you checked Layout Adjustment on the Control Panel (shown in your screen shot).
In CS6, Layout Adjustment was denigrated but has not disappeared. The preferred method is Liquid Layout which can be more flexible. However, you can still turn it on. You just do it in the Layout > Margins and Columns dialog.
You could also do some reading in the documentation and Adobe TV videos and try playing with the new Liquid Layout features.
Great! Good luck.