Photoshop INCLUDES a "grow" selection but NOT a "Shrink" selection!!! NOT ONLY THAT, the Grow selection DOESN'T EVEN allow you to input number of pixels to grow by!!! I CANNOT believe that something SO OBVIOUS was overlooked. fyi, Gimp has ALL THIS
The commands you want have actually been in Photoshop a very long time. Make a selection and click Select > Modify > Expand, where you can then enter the number of pixels to grow. Similarly, use Select > Modify > Contract to enter the number of pixels to shrink.