well free icons sets are usually crap, but...
If the designer had any consideration s/he arranged the set in a grid. so there should be X icons across by Y icons down.
Say that that's 5 across by 6 down ( so X=5, Y=6);
you want each icon to be 80pxs X 80pxs, in including white space.
open the file in PS, when it asks what size( resolution) you will enter X x iconWidth, Y x IconHeight ( in this example: 5*80=400, 6*80=480, so 400 and 480)
When the file opens in PS all the icons should have been resized to the size you want. Of course they are still all together so we need a couple of more steps.
Place a vertical guide every icon width (so in our example every 80px horizontally) and a horizontal guide every icon height (so in our example every 80px vertically)
Click on the slice tool, on the tool panel, click on slice from guides.
Go to save for web... and save slices.. Voila!
Do keep in mind that reducing an icon's size might make it unrecognizable even if you started from the vector original.
Of course , if all you graphics are not arranged in a consistent grid or have different dimensions then you are SOL. the key to efficient automation is consistency!