can someone tell me how to change the size of all objects? I need to divide the actual size by 10 to get a printed version of the model.
Maybe you can start here:
And also:
https://help.sketchup.com/en/sketchup/scaling-your-model-or-parts-your-model
Try the Scale tool.
Or export your model to STL in cm and import it into your slicer in mm.