Sets the height of the arrow cap
#include <GDIPlus.au3>
_GDIPlus_ArrowCapSetHeight ( $hArrowCap, $fHeight )
| $hArrowCap | Handle to a ArrowCap object | 
| $fHeight | Specifies the length, in units, of the arrow from its base to its point | 
| Success: | True. | 
| Failure: | False and sets the @error flag to non-zero, @extended may contain GPSTATUS error code ($GDIP_ERR* see GPIPlusConstants.au3). | 
Search GdipSetAdjustableArrowCapHeight in MSDN Library.
#include <GDIPlus.au3>
#include <GUIConstantsEx.au3>
Example()
Func Example()
    Local $hGUI, $hGraphic, $hPen, $hEndCap
    ; Create GUI
    $hGUI = GUICreate("GDI+", 400, 300)
    GUISetState(@SW_SHOW)
    ; Create resources
    _GDIPlus_Startup()
    $hGraphic = _GDIPlus_GraphicsCreateFromHWND($hGUI)
    $hPen = _GDIPlus_PenCreate(0xFF000000, 4)
    $hEndCap = _GDIPlus_ArrowCapCreate(3, 6)
    _GDIPlus_PenSetCustomEndCap($hPen, $hEndCap)
    ; Draw arrow 1
    _GDIPlus_GraphicsDrawLine($hGraphic, 10, 120, 390, 120, $hPen)
    ; Draw arrow 2
    _GDIPlus_ArrowCapSetHeight($hEndCap, _GDIPlus_ArrowCapGetHeight($hEndCap) + 2)
    _GDIPlus_PenSetCustomEndCap($hPen, $hEndCap)
    _GDIPlus_GraphicsDrawLine($hGraphic, 10, 150, 390, 150, $hPen)
    ; Draw arrow 3
    _GDIPlus_ArrowCapSetHeight($hEndCap, _GDIPlus_ArrowCapGetHeight($hEndCap) + 2)
    _GDIPlus_PenSetCustomEndCap($hPen, $hEndCap)
    _GDIPlus_GraphicsDrawLine($hGraphic, 10, 180, 390, 180, $hPen)
    ; Loop until the user exits.
    Do
    Until GUIGetMsg() = $GUI_EVENT_CLOSE
    ; Clean up resources
    _GDIPlus_ArrowCapDispose($hEndCap)
    _GDIPlus_PenDispose($hPen)
    _GDIPlus_GraphicsDispose($hGraphic)
    _GDIPlus_Shutdown()
EndFunc   ;==>Example