This is not what I intend (I knew about this other post). What I want is to keep it one big diagram (no need to split it up); I just want that the word-properties of the image (width and height (in cm) in the document are adjusted. Word will then scale the image by itself; no complicated manipulations!

Example link where a similar thing is realized using scripting within word:
http://en.allexperts.com/q/Microsoft-Word-1058/Word-resize-pictures.htm