A mountain animal is not of necessity any particular size. However, the Lhasa's function as an indoor sentinel would suggest a handy size. In fact, the Lhasa is about as small as a dog could be and still be viable in the Tibetan climate. The smaller the linear dimensions of an animal, the greater the surface area is in proportion to the mass. This means that a very small animal has a much greater surface (from which to lose heat) in comparison to its mass, (which produces heat). These very little animals usually have very high metabolisms, live in protected burrows, and hibernate through unfavorable conditions. None of these adaptations are available to canines.

The outdoor showring tends to exaggerate the length and cut the apparent height of dogs, especially small dogs. Breeders have responded to this pressure by selecting for a taller,  squarer dog. The height of most of the Lhasas competing in Western countries has increased to the point where a correct sized Lhasa often looks abnormally small.