I am thinking of buying a viola so I'm investigating things. I found that there are different sizes for violas so does tha mean every viola needs different sized strings. Thanks

Unlike other bowed string instruments, the viola has not been standardized. When you buy or rent a viola, you'll have to decide what size works best for you (physiologically and aesthetically) and buy appropriate strings. A viola proportionate in size to its range, relative to the violin, would be too big to hold under your chin. Thus all violas are compromised to some extent, and the larger the viola, the less compromised (and the greater the stretches for the fingers). On the other hand, it is just this disproportion that gives the viola its characteristic timbre.

No. They are pretty standard.

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Edit: My bad, the answer is yes, according to scotch (see below), there are different sizes.

Samsmusic: You obviously haven't the slightest idea what you're talking about and appear to be randomly and pointlessly replying to missives merely to rack up points or some such thing. You're wasting your own time and everyone else's. Here is what juststrings.com which sells viola strings says about the matter: "Violas are available in many more sizes than the violin. There are 11, 12, 13, 14, 14.5, 15, 15.5, 16, 16.5 and 17 inch violas (measured as the length of the back of the body, not including the neck). As such, many different sizes of viola strings are available to fit both student and extra long models."

Two more points: 1) The proportion of the neck size to the back size on a viola can vary as well. 2) Since, obviously, the strings are wound around the tuning pegs, the same length of string can work (at least more or less) on a range of viola sizes.

Thank you for the information. I have learned something.