I would say there is a clear lack of options when it comes to create pre-world war II ruins, unless you want a medieval castle ruin, but if you want something smaller like the ruin after an abandoned cottage/croft its not as easy.
It can definitely be done, here's my attempt.
is it possible to change the map sizes (Terrain Size/Playable Area) of an already existing/published map via worldbuilder? Or is stamping into a newly set up map the only solution?
It is very obvious that they did this patch because they did not have time to do a "real" patch so they just apply some quick fixes to bring balance into a better state. Then they go back to working on new SP, obs mode, etc. I think they are barely making any effort to bugs and balance at all.
With the stamp tool you are able to copy custom objects, water, splines, splates, heights ect. in a selected part of a map. You can paste ("stamp") this selection into the same or in a different map as much as you like to.
Edit:
You can find it at the maintoolbar in worldbuilder under "Stamp Placement", 3rd icon right from "Water Placement".