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.