My man (he teaches fiddle) says you can figure out the right size thus: with the fiddle under chin in playing position, the player should be able to reach along underneath the neck with left arm and curl fingers round the scroll on the head, so that the fingertips are touching the front of the pegbox, or at least most of the way round the scroll. Mind you I started lessons at school aged 9 on a full size fiddle and I managed, although I can see how it would have been easier with a smaller one.