Insert the objects you want to scale inside a new Panel object or any other graphical object.

Create a Panel object by either using the “Graphical objects” tool, or the menu “Objects”->”New”->”Panel”.

Move the objects inside the panel:  With the mouse, drag the thumbnails of each of the objects you want to group together, on top of the thumbnail of the panel object you just created. You find these thumbnails in the “Objects” panel.

Now just select the panel object and drag the control points to scale, or change the input fields in the “Object” panel.