OK, Sorcha, here's a little more. Lois the violin teacher who ususally plays a viola stopped in for a chiropractic treatment this morning, and I wouldn't let her see my wife the chiropractor until she gave at least somewhat of an explanation.

She says that a cello is proportionally larger than a violin. A viola is larger than a violin, but the increase in size is not proportional - otherwise, it wouldn't fit under your chin. You can fit viola strings on a violin and get a reasonable approximation of the sound, but you need the larger and differently-shaped sound box of the viola to get the characteristic viola timbre. Generally, violas are thicker and wider than violins, but not all that much thicker and not all that much wider. Lois has a favorite viola that is only a half-inch wider than her largest violin.

One difference that is noticeable is the scroll, which is quite a bit larger on a viola.

This all seems to jive with the information John picked up and posted above.

-Joe-