The Department of Defense, the National Security Agency/Central Security Service, and other agencies have published standards which shredders must comply with to earn the approval of the various agencies: the standards apply to destruction of all top-secret or otherwise classified papers and documents.
The DOD document 5220.22-M, Chapter 5, Section 7, states that "Crosscut shredders shall be designed to produce residue particle size not exceeding 1/32 inch in width (with a 1/64 inch tolerance) by 1/2 inch in length". An additional requirement is that there is more than one output bin, so that the various documents get scattered as well as shredded.
NSA/CSS standards for the destruction of secret documents are tougher than that. The DOD standard shredder will convert an 8 1/2 x 11" sheet into 6,093 tiny pieces; but the NSA/CSS requirement will produce no less than 12,065.5 pieces! (hey, we're quoting: www.papershreddersinfo.com/highsecurityshredders.htm) The standard (specification 02-01) includes the following requirements:
Note that the second requirement implies that particles will typically be 1mm x 5mm: most of the manufacturers describe their compliance in this fashion.
The NSA/CSS document EPL-02-01-M_APR05.pdf (April 2005) lists a large number of copiers that meet the specifications. The document is updated on a quarterly basis; it stipulates that mention on the list does not constitute an endorsement. Most of the major manufacturers are listed: Capital Shredder Corp., Cummins-Allison, Dahle, Ecco, Fellowes, HSM/Geha, Rockville/Marquis, Olympia, SEM, Whitaker/Destroyit/Intimus, and others. The machines are rather more expensive than non-DOD-compliant models, and some require special environments, such as a dedicated electrical connection.