Since 1952 the American Shetland Sheepdog Club has asked for a show Sheltie between 13 and 16 inches at the shoulder. Still, size is one of the most tenacious problems plaguing the breeder today, with both oversize and undersize Shelties appearing, sometimes in the same litter. The Sheltie is still a young breed with many kinds of dogs in its background. Often the oversize animals strongly resemble their Collie ancestors, while the undersize ones may exhibit Pomeranian characteristics.
(text from the book Sheltie Talk, by Betty Jo McKinney and Barbara Hagen Rieseberg)
Size is measured at the highest point of the shoulder blades, just behind the base of the neck. The dog should be measured standing on a hard, level surface with the front legs vertical and the head in a natural position. The ideal measuring device is an adjustable wicket, but for the owner of a single Sheltie, the easiest way is to tape a yardstick to a wall with the 0" mark against the floor. Then take a drawing triangle or a rectangle of cardboard (the cardboard backing from a pad of paper works fine) and hold an edge against the yardstick above the height of the dog. Stand the dog with its front feet lined up with the yardstick and slide the triangle or rectangle down until it just rests on the withers. Read the dog's height from the yardstick at the bottom edge of the cardboard.
(text from the web page Sheltie Size, by Sue Ann Bowling)
Several growth charts have been developed over the years to estimate adult size from measurements taken at various stages in a puppies life. For more information on sheltie size and size charts visit the web page "Sheltie Size" at http://bowlingsite.mcf.com/Size/size.html.