Guest Desktop size changes each boot

Hi, a problem in Leap, which I dont see e.g. in a 13.1 guest on VBox:
Once the guest is fully booted, pull the Guest window up to the desired size.
13.1: After reboot, Guest window has the same size as before
Leap: Guest window comes up in small (800x600?) size, need to resize manually.
Is there a trick how I can make the size of the guest window persistent?