Quote:
CB_SETITEMHEIGHT
An application sends a CB_SETITEMHEIGHT message to set the height of list items or the selection field in a combo box.
To send this message, call the SendMessage function with the following parameters.
SendMessage(
(HWND) hWnd, // handle to destination window
CB_SETITEMHEIGHT, // message to send
(WPARAM) wParam, // item index
(LPARAM) lParam // item height
);
Parameters
wParam
Specifies the component of the combo box for which to set the 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 wParam parameter is the zero-based index of a specific list item.
lParam
Specifies the height, in pixels, of the combo box component identified by wParam.
Return Values
If the index or height is invalid, the return value is CB_ERR.
Remarks
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.
...from MSDN (again)