Hi James /Ilpo,
Sorry for the long delay in getting back to you.
The treeview placed inside the drop down of the combo box cannot be resized on dragging itself. You can make use of the PopupControlContainer to resize it. Please refer to the attached sample which illustrates the resizing of PopupControlContainer:
http://websamples.syncfusion.com/samples/Tools.Windows/F70986/main.htmKindly let me know if you have any other questions.
Regards,
Geetha