The shred size is the width of the paper after it has been shredded. On a strip-cut shredder a 1/8" or 1/4" shred width is most common. As a rule, the smaller the shred width, the more difficult it becomes to reconstruct the document.
A cross-cut shred size is listed on the specification as two measurements, representing the width and length of the particles. Commercial cross-cut shredders will vary in the particle size with each manufacturer. High security cross-cuts,however, are built to meet a strict Department of Defense standard of 1mm x 5mm. Dahle's high security models exceed this standard (1mm x 4mm).