Expand or contract a selection by a specific number of pixels
You
can use commands in the Select menu to increase or decrease the
size of an existing selection and to clean up stray pixels left
inside or outside a color-based selection.
Use a selection tool to make a selection.
Choose
Select > Modify > Expand or Contract.
For Expand By or Contract By, enter a pixel value between
1 and 100, and click OK.
The selection border is moved outward or inward by the
specified number of pixels. Any portion of the selection border
that runs along the canvas edge is unaffected.