An application sends the CB_SETDROPPEDWIDTH message to set the maximum allowable width, in pixels, of the list box of a combo box with the CBS_DROPDOWN or CBS_DROPDOWNLIST style.
CB_SETDROPPEDWIDTH wParam = (WPARAM) wWidth, // width of list box, in pixels lParam = 0, // not used, must be zero
If the message is successful, The return value is the new width of the list box.
If the message fails, the return value is CB_ERR.
By default, the minimum allowable width of the drop-zdown list box is 0. The width of the list box is either the minimum allowable width or the combo box width, whichever is larger.