Although it is easy to use Select > Modify in order to expand or contract a selection by a specific number of pixels, for additional control, try using the Maximum and Minimum filters. To contract or expand a selection by a decimal number (not a whole number as is the limit for the Select > Modify command), first, make your selection, then click the Quick Mask icon to view the red overlay before selecting the filter (otherwise the filter will effect the pixels in the photo that you have selected). Then, choose Filter > Other > Minimum to contract the selection by a non-whole number or choose Filter > Other > Maximum to expand the selection by a non-whole number.
Note: both of the filters are looking at the values of gray within the specified radius that you define. The Preserve Roundness option will help keep round shapes round instead of being reduced using a more “rectangular” method which will cut corners when contracting. The Preserve Squareness will help keep rectangular shapes with more square edges from getting rounded. Both filters can be used for choke and spread operations on masks or images (removing dirt, enlarging bright points, etc.).