Open your file and make a duplicate layer which should be the active layer.
Click on the crop tool.
Make sure the perspective box is not checked; otherwise the hide box will be grayed out.
Enter the size to which you wish to crop in the appropriate boxes on the options bar at the top of the screen.
Make sure there is a check in the "hide" box. If save save your file as a PSD (and that is VERY IMPORTANT to save the layers)you will be able to recrop the image to different dimensions that next time you open the psd file and you won't have to re-edit the picture.