Like the poodle and the Three Bears, the schnauzer comes in three sizes, but unlike the poodle (or Goldilocks's bears), each size is a separate breed. The standard or medium-size schnauzer is the original and the two other breeds, the giant and miniature, were created by crossing with other breeds.
Wait until your puppy stops growing. For miniatures this is at about 6 to 12 months, for standards 10 to 16 months and for giants at up to 2 years of age.
Measure him and see which size group he lands in. The AKC breed standard height at the shoulder for miniatures is 12 to 14 inches, for standard males 18 to 20 and females 17 to 19 inches, and for giants 23 1/2 to 27 1/2 inches; any schnauzer that falls outside these measurements is not eligible.
Love him, however tall or small. Unless you cherish ambitions as a breeder, if your pup doesn't qualify for the show ring, who cares?
Gather up a carpenter's level, a measuring tape or yardstick, a pencil and a helper.
Stand the dog on a level surface next to a blank wall in the stack position—this is where you may need a helper to steady him.
Lay the level across the high point of the dog's withers (where the neck joins the shoulders) with one end touching the wall.
Raise or lower the wall end of the level until the bubble is perfectly centered and pencil-mark the wall at the lower surface of the level.
Measure from the floor to the mark on the wall and you have a reasonably accurate height for your dog. Measuring at a dog show is done with a special apparatus.
Have your pup tested genetically to determine his breed. Miniature and giant schnauzers are on the list of dog breeds most canine genetic testing labs can identify, but for some reason the standard schnauzer is not—at least not as of August 2012.
Get your vet to take the sample or send for a kit and do it yourself—all it takes is a cheek swab from the dog. Mail the kit to the lab and it will send you a report. The cost usually runs somewhere between $30 and $150, paid in advance.
Bear in mind that the accuracy of genetic testing is hotly debated, some experts swearing it's as reliable as an atomic clock and others that it's a waste of money.