Get the image width
#include <GDIPlus.au3>
_GDIPlus_ImageGetWidth ( $hImage )
| $hImage | Handle to an image object | 
| Success: | an image width, in pixels. | 
| Failure: | -1 and sets the @error flag to non-zero, @extended may contain GPSTATUS error code ($GDIP_ERR* see GPIPlusConstants.au3). | 
Search GdipGetImageWidth in MSDN Library.
#include <GDIPlus.au3>
#include <ScreenCapture.au3>
#include <WinAPIHObj.au3>
Example()
Func Example()
    Local $hBitmap, $hClone, $hImage, $iX, $iY
    ; Initialize GDI+ library
    _GDIPlus_Startup()
    ; Capture 32 bit bitmap
    $hBitmap = _ScreenCapture_Capture("")
    $hImage = _GDIPlus_BitmapCreateFromHBITMAP($hBitmap)
    ; Create 24 bit bitmap clone
    $iX = _GDIPlus_ImageGetWidth($hImage)
    $iY = _GDIPlus_ImageGetHeight($hImage)
    $hClone = _GDIPlus_BitmapCloneArea($hImage, 0, 0, $iX, $iY, $GDIP_PXF24RGB)
    ; Save bitmap to file
    _GDIPlus_ImageSaveToFile($hClone, @MyDocumentsDir & "\GDIPlus_Image.bmp")
    ; Clean up resources
    _GDIPlus_ImageDispose($hClone)
    _GDIPlus_ImageDispose($hImage)
    _WinAPI_DeleteObject($hBitmap)
    ; Shut down GDI+ library
    _GDIPlus_Shutdown()
    ShellExecute(@MyDocumentsDir & "\GDIPlus_Image.bmp")
EndFunc   ;==>Example