Hi Rainer !
Thank you for your message!
It is possible to change the size of multiple objects at once. In order to do so, you'll need to press and hold Ctrl on your keyboard + left-click on the objects. This way, all the objects will be selected and by changing the size of one of them, other objects will be resized as well.
Hope this helps.
But its not the expected result... have a look:
i just want to resize the whole thing to a little smaller size using a group
This is how it works in PPT