An application sends a CB_SETITEMHEIGHT message to set the height of list items or the selection field in a combo box.
CB_SETITEMHEIGHT wParam = (WPARAM) index; // item index lParam = (LPARAM) (int) height; // item height
This parameter must be -1 to set the height of
the selection field. It must be zero to set the height of list items, unless
the combo box has the CBS_OWNERDRAWVARIABLE style. In that case, the index
parameter is the zero-based index of a specific list item.
If the index or height is invalid, the return value is CB_ERR.
The selection field height in a combo box is set independently of the height of the list items. An application must ensure that the height of the selection field is not smaller than the height of a particular list item.