1 person found this helpful
Use Rectangular Marquee in Fixed Size mode to numerically specify a region then command Image > Crop.
Notice that was "command Image > Crop" and not "Crop Tool".
That'll discard the cropped-away pixels, though, and not merely hide them.
If you have CS6 and want to keep and hide these pixels then activate Crop Tool after defining the marquee; Ps will snap the crop boundary to the marquee and you can disable the option to delete pixels.
In addition to what Conroy said. if you have an active rectangular selection, you can Selection > Transform Selection, and precisely enter the size and position in the Option Bar. Then Image > Crop.