The AKC Official Sheltie Standard for showing is 13-16 inches at the wither. These dogs are usually 20-40 lbs. Because larger dogs may have more 'presence' in the show ring, many show breeders breed for Shelties on the high side of the standard, many of these dogs 'go over', i.e. grow larger than 16 inches. Today we see many 17-18 inch 'pet' shelties who may weigh 40-55 lbs. Standard Sheltie breeders will view the dogs who are closer to 14 inches as being 'small'. These same breeders frown greatly upon breeders who specialize in 'undersize' shelties. Make no mistake, there is no separate AKC registry for Toy or Mini Shelties, there is only one AKC Shetland Sheepdog registry.
Because some breeders and Sheltie lovers were taking an interest in smaller type (undersize) Shelties, and because these same breeders wanted to keep the quality in the smaller Shelties, the Toy Sheltie Club of America was formed. These breeders wanted to be recognized as responsible, caring breeders who had the goal of beautiful, healthy, smaller Shelties than those that the show breeders were producing.
The Toy Sheltie is classically 10-12 1/2 inches at the withers and 8-15 lbs. Very few AKC Shelties fit this size description and even fewer are smaller than this. Because so many of today's buyers have little interest in conformation showing and have very busy lifestyles that they would love for their pets to be a part of, the Toy Sheltie type is rapidly growing in popularity. These dogs are still eligible to compete in AKC Agility, but would be disqualified from the conformation ring because of their diminutive size.
I will point out whether you choose a Standard size or a Toy or Mini size Sheltie, once you have it spade or neutered , it will tend to gain 5 (with the Toy Shelties) or more (with standard Shelties) pounds and its coat will become much fuller (especially females). Bella was a 12 inch 10 lb sparsely coated girl until I had her spade. After 'her surgery' she gained 7 pounds and tons of beautiful coat. Bella prefers a more inactive lifestyle (more walks and exercise would undoubtedly help), but she still remains within that 'magic' 'Under 20 lbs' number today. Be advised that you 'unfixed' sheltie may be lighter in both weight and coat than the same sheltie 'fixed' and ultimately 'size' is determined by height not weight. We encourage 'fixing' your puppy, please don't freak out if his/her weight is a bit more than the parents. Many of Frosty's (10 lbs) sons have reached 12-15 lbs after they were neutered, this is completely normal. Also take 'type' into consideration before you panic about weight. I have seen spindly (unattractive) 12 pound 14 inch shelties, and beautiful 12 pound 11 1/2 inch shelties (like Cassie). Judging a dog just on height or weight is short sighted. A balanced athletic individual is most desirable. Notice if a dog is of heavier or lighter 'bone' (bones in the face and legs), a lighter boned sheltie might be a great lapdog, but you may need more bone for agility or to work around your farm. Talk to your breeder about what you desire out of your new partner so he/she can help you find the perfect match in both type and temperment.
How did Sheltie Size become so variable
Shetland Sheepdogs are one of the few breeds with an absolute disqualification on size. At the same time, the breed as it now exists has within the last century and a half or less combined breeds ranging in size from Papillons, Pomeranians and English Toy Spaniels to full-sized Collies. The allowed size range has changed during the development of the breed, generally moving upward from a one-time maximum of 12" as more and more Collie genes were incorporated. (The actual sizes were often much larger, especially during the period when Collie crosses were common.) But the desired size range is achieved by balancing genes from large and small breeds, and consequently breedings in which both parents are the correct size can produce puppies much larger or smaller than desired. As one result, Shetland Sheepdog breeders tend to become obsessed with measuring the size of their puppies, and a number of growth charts have been developed to estimate adult size from measurements made at various ages. See Growth Charts.
How to Measure Size/Height
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 or guillotine standard, 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.