Cropping to a Specific Size
If you would like to crop to a specific size with the Crop tool (C), enter the dimensions and resolution you would like in the tool options up in the Options Bar. When you click-and-drag out your crop border, you will automatically constrain the crop handles to the size that you have set.
This just doesn’t work. Set crop constraints and the crop handles are only constrained by the aspect ratio. The specific dimensions are ignored and the image is resized to the aspect ratio, rather than being limited to the specific crop size.