CB_SETHORIZONTALEXTENT 
An application sends the CB_SETHORIZONTALEXTENT message to set the width, in
pixels, by which a list box can be scrolled horizontally (the scrollable width).
If the width of the list box is smaller than this value, the horizontal scroll
bar horizontally scrolls items in the list box. If the width of the list box
is equal to or greater than this value, the horizontal scroll bar is hidden or,
if the combo box has the CBS_DISABLENOSCROLL style, disabled.
CB_SETHORIZONTALEXTENT
wParam = (WPARAM) cxExtent; // scrollable width of list box, in
pixels
lParam = 0; // not used, must be zero
Parameters
cxExtent
Specifies the scrollable width of the list box, in pixels.
Return Values
No return value.
See Also
CB_GETHORIZONTALEXTENT