ComboBoxes uses another skin section if style is DropDownList.
Used section is ‘COMBONOEDIT’ (or ‘BUTTON’ if ‘COMBONOEDIT’ doesn’t exist).
Sizing problems exists in comboboxes derived from standard VCL. The reason is in the ItemHeight property which doesn’t allow to change height of the control arbitrarily.
I’m planning to make a full new ComboBox component soon.