Once you are inside the canvas (already fully of elements) you must move one by one if something is wrong. Nothing of this is made automatically.
For example: I add an image with a size, then, I add labels, buttons, and forms. I made everything with many sizes as possible, but I must change the first size of every single component, the image. Once changed none of those components (the other ones) are gonna move to replace the space left from the change of the image's size.
I believe, this is like an absolute position in CSS. You must change it manually and you can't change the sizes without concerning about the design you are gonna left behind.
PowerApps needs to order elements automatically at time to start modifying and changing parameters inside each one.