Bava Batra 14 - Ideal Dimensions for a Torah Scroll
The Torah scroll should preferably be written on thick parchment, and its ideal proportions are such that its height and circumference are the same, and the height is six hand-breadths, because that was the size of the Tablets on which the Ten Commandments were engraved.
Rav Huna wrote seventy Torah scrolls in his lifetime, and only one turned out to be a scroll whose height and circumference were the same. Rav Acha wrote one Torah scroll on calfskin (which is thicker and thus it is harder to make it ideal size), and it turned out to be a scroll whose height and circumference were the same. The Rabbis cast their eyes on him in disbelief and he died.