For a given instrument (violin, viola, cello, bass is more complicated), the tuning of a fractional size instrument (1/16, 1/8, 1/4, 1/2, 3/4 are commonly seen, 7/8 used to be frequently called "lady's cello") is always the same as the full size instrument.
viola is a bit complicated because professionals have different sizes (15', 16', sometimes 17' for body length, remove about one inch for string length), so that one refers to the body length and string length (the last one being the more important for buying strings).
A good rule for chosing one's viola size is : are you able to confortably stop on each string the first tone after the empty string with the index as well as the fifth interval with your little finger ? (For the A-string : try to stop a B and a E, for the C-string : try to stop a D and a G).
The size matters for the sound, but a lot less than the quality of construction. A good violin maker will change the internal measurements, the position of the f-holes, to maintain the best body resonance for the intended tuning. I have played several good 1/2 cellos that sounded a lot better and more powerful than ordinary full size ones.
The only reason to buy a fractional size is the age and body measurements of the player, not the sound. Also make sure that the player will have time to play and study say two years on this instrument. This is not about economy, just so that he/she will develop enough is ear and finger skills, so that the transition to a larger one will be really easy.