yep,Daisy is bang on-there is no such thing as a 'standard' sized viola and they come in many shapes too.

the problem with the viola is that the dimensions of even the larger instruments(17-17-1/2) are not in proportion to the tones and so is 'slower to speak'.this is why viola players have to work a lot harder than their violin brethren in producing a decent sound.of course,the advantage is that rich sound and the fact that getting a sound from a violin is ridiculously easy once you've made a viola speak!plus,what is good for sound production on viola is also ok for the violin-the reverse is n't always the case as you can get away with a lot more on the violin!

i would want to have a word with the teacher who sold your pupil that instrument as a viola and if she wants to stick with viola i'ld suggest she tries to get a real one as soon as.

as for treble clef,yes,you have to learn it but that gives us violas another advantage of our poorer violin playing friends!
if you want to look into it a bit more i would recommend two books especially: 'The Viola' by Henry Barrett(ISBN:0-8173-6402-1) which also has a section specifically for violinists changing to viola and alto clef,but it's expensive (in the uk anyway) so maybe the library for that one and a very good read indeed is David Dalton's 'Playing The Viola' (conversations with William Primrose,ISBN:0-19-816195-6) and this has a good deal of hard information as well as some very dry asides and i can't speak of it highly enough!
best wishes anyway