Argh - stupid me. Of course I should have looked into the vtour.config
Thank you Hans, for pointing me into the right direction and the others for testing.
Got it working now. The general size of the thumbs is determined in the vtour.config and the displayed size is determined in the vtour.xml
If you don't want them square just enter the desired size. Be aware that there a three different states: When the tour starts, when they are collapsed and when they are "spread". For each state you can define a specific size. Just make sure that the setting in the config matches the maximum size you want to use in the xml.
Best
Jürgen