Solution: Draw a rectangle to the size of the page without margins. Let's say your page is a DIN A4 page 210 mm wide and 297 mm height, with a margin of 10 mm each side.
So the width of your workspace is 190 mm, the height 277 mm, and the new rectangle will cover your workspace.
Now select all elements with your mouse and GROUP them.
Then go to PROPERTIES, click the chain to keep the aspect ratio and type the new size of 100 mm for the width.
The group will rescale, and all enclosed elements will be scaled accordingly.
Now select the group and right click -> UNGROUP.
Pretty straight forward and simple.