You can resize pages like that rather easily using InDesign's Liquid Layout behaviors. I've written 4 or 5 tutorials about Liquid Layout. Here's one: http://www.digitalartsonline.co.uk/tutorials/adobe-indesign/resize-page-using-indesign-cs6 s-liquid-layouts/
How exactly do I resize a cover-size from A4 to A5, using Indesign’s Liquid Layout behaviours. A step by step guide would be very much appreciated. I am still a beginner in Indesign...
That article IS a step-by-step guide. In fact, being that I wrote it for a British magazine, it specifically tells you on page 1, step 1 that it will walk you through changing A4 to A5.
I would transform the page—when you do that the Liquid Layout rule handles any change in aspect ratio. A5 and A4 have almost the same AR so you can use Scale as the rule.
Here I have the Page Tool selected (the page has selection handles) with the Liquid Layout rule set to Scale
All I have to do is choose A5 from the custom page pop-up:
Is it possible you post a video while you doing all these steps? Would be very glad.
Is it possible you post a video while you doing all these steps?
No, but for a single page like a bookcover it would only be a few simple steps.
With your page selected in the pages panel:
•Select the Page tool from the Tools panel (3rd tool from top).
•Make sure your Control Panel is open (Window>Control)
•From your Control panel choose Scale under Liquid Layout
•And then change the page size to A5 from the pop-up that is to the left of Liquid Layout