Another approach might be to go to the left or right master page and use the page tool AND click on the reference dot system before changing the page size. For left page, you might want to click the upper right corner reference dot system, and for a right hand page, you might want to click on the upper lefthand reference dot system.
When you change the Width and Height in Document Setup the page items keep their relationship to the spine for facing page documents and remain centered for non-facing page documents:
If you want to anchor the page items to a specific Reference Point you can use the Transform Panel and Pages tool to pin the items as Mike suggests. Here I have two pages selected in the Pages panel with the Transform Reference Point set t0 UpperLeft: