Hi Falk,
Thank you for using Syncfusion products.
You can use the property PreferredChildPanelHeight of XPTaskBarBox to set the client size.
Please refer the following sample that displays all GroupViewItems in the XPTaskBarBox.
http://websamples.syncfusion.com/samples/Tools.Windows/F73531/main.htmPlease let me know if this helps you.
Regards,
Jaya