Dremel offers three different size sharpening stones that can be used with this attachment.  We have a 5/32'' stone (#453 – bluish/green in color), a 3/16'' stone (#454 – brownish/red in color), and a 7/32'' stone (#455 – pink in color).  These stones are sized in the same way that an actual sharpening file would be sized.  When in doubt, refer to the specific owner’s manual for your chain/chain saw.  That should be able to provide the proper recommendation for the size file /stone you will need to successfully sharpen your chain.