The viola looks like a large violin and in terms of its construction, it is more or less the same. In common with all string instruments, there are no fixed dimensions for a viola and both players and makers will have their own preferences. The 'bigness' of the sound produced has much more to do with the woods and varnish used than the actual size of the instrument (the player makes quite a difference too!). Different types of wood such as sycamore, spruce and maple are carefully selected for their grain and density.
One of the most important factors in determining the sound quality of the instrument is the varnish, which at first comes as a surprise to many people. The reason the varnish is so important is that it covers the entire exterior of the instrument, bonds with the surface of the wood and it is from this shiny surface that the sound of the instrument resonates. So the quality of the varnish can greatly effect the way the instrument resonates.