An application sends a CB_GETITEMHEIGHT message to determine the height of list items or the selection field in a combo box.
CB_GETITEMHEIGHT wParam = (WPARAM) index; // item index lParam = 0; // not used; must be zero
This parameter must be -1 to retrieve the height of the selection field. It must be zero to retrieve 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.
The return value is the height, in pixels, of the list items in a combo box. If the combo box has the CBS_OWNERDRAWVARIABLE style, it is is the height of the item specified by the index parameter. If index is -1, the return value is the height of the edit control (or static-text) portion of the combo box. If an error occurs, the return value is CB_ERR.