The viola is similar in material and construction to the violin. A full-size viola's body is between 25 mm (1 in) and 100 mm (4 in) longer than the body of a full-size violin (i.e., between 38 and 46 cm [15–18 in]), with an average length of 41 cm (16 in). Small violas typically made for children typically start at 30 cm (12 in), which is equivalent to a half-size violin. For a child who needs a smaller size, a fractional-sized violin is often strung with the strings of a viola. Unlike the violin, the viola does not have a standard full size. The body of a viola would need to measure about 51 cm (20 in) long to match the acoustics of a violin, making it impractical to play in the same manner as the violin. For centuries, viola makers have experimented with the size and shape of the viola, often adjusting proportions or shape to make a lighter instrument with shorter string lengths, but with a large enough sound box to retain the viola sound. Prior to the eighteenth century, violas had no uniform size. Large violas (tenors) were designed to play the lower register viola lines or second viola in five part harmony depending on instrumentation. A smaller viola, nearer the size of the violin, was called an alto viola. It was more suited to higher register writing, as in the viola 1 parts, as their sound was usually richer in the upper register. Its size was not as conducive to a full tone in the lower register.
Several experiments have intended to increase the size of the viola to improve its sound. Hermann Ritter's viola alta, which measured about 48 cm (19 in), was intended for use in Wagner's operas. The Tertis model viola, which has wider bouts and deeper ribs to promote a better tone, is another slightly "nonstandard" shape that allows the player to use a larger instrument. Many experiments with the acoustics of a viola, particularly increasing the size of the body, have resulted in a much deeper tone, making it resemble the tone of a cello. Since many composers wrote for a traditional-sized viola, particularly in orchestral music, changes in the tone of a viola can have unintended consequences upon the balance in ensembles.
One of the most notable makers of violas of the twentieth century was Englishman A. E. Smith, whose violas are sought after and highly valued. Many of his violas remain in Australia, his country of residence, where during some decades the violists of the Sydney Symphony Orchestra had a dozen of them in their section.
More recent (and more radically shaped) innovations have addressed the ergonomic problems associated with playing the viola by making it shorter and lighter, while finding ways to keep the traditional sound. These include the Otto Erdesz "cutaway" viola, which has one shoulder cut out to make shifting easier; the "Oak Leaf" viola, which has two extra bouts; viol-shaped violas such as Joseph Curtin's "Evia" model, which also uses a moveable neck and maple-veneered carbon fibre back, to reduce weight: violas played in the same manner as cellos (see vertical viola); and the eye-catching "Dalí-esque" shapes of both Bernard Sabatier's violas in fractional sizes—which appear to have melted—and David Rivinus' Pellegrina model violas.
Other experiments that deal with the "ergonomics vs. sound" problem have appeared. The American composer Harry Partch fitted a viola with a cello neck to allow the use of his 43-tone scale. Luthiers have also created five-stringed violas, which allow a greater playing range.