Maybe try use Liquid Layout - Based on Master and then adjust size of your pages/masters
Top menu - Layout and then Liquid Layout - try it
Unless I am misunderstanding you the page doesn't change size just the orientation of the placeholders
I've been looking at this, I think this isn't what I am looking for. As the page sizes remain the same throughout.
I was thinking that If I have an image as a portrait when I then drag a Master over that page it would then apply that image to the new place holder, instead it over lays another placeholder, with the old image remain.
Another example is in lightroom books.
Please see the video here
Anyone have any idea if this is possible?