Dede's most used and most favorite are the Blue 2 and Purple 4.  The inches is the amount of hair left on the dog.  All Sales Final. It fits the Andis Ultra Edge and Ceramic Edge Blades.  Not sure, ask first, if you have a cheap clipper that came with plastic guards, this will likely not work, but better safe than sorry and call in or email a question before you purchase. 
With many stores sold out of this product and Wahl not shipping at full capacity, limited supplies mean higher demands.  Get it while you can.