When you have made a selection in Photoshop, you can easily make that selection a few pixels larger of smaller by choosing Select > Modify. To make your selection bigger choose Expand, to make the selection smaller, choose Contract. It works best if you Expand or Contract by only a small number of pixels at a time (i.e. 4 or 5 pixels at most). Otherwise the edges of the selection won’t stay sharp, they have a tendency to look quite bumpy.