Charles Ellicott Commentary Exodus 28:16

"Foursquare it shall be [and] double; a span shall be the length thereof, and a span the breadth thereof." — Exodus 28:16 (ASV)

Foursquare it shall be. —On the idea of perfection connected with the square, see Note on Exodus 27:1. But for this, twelve gems would probably have been arranged in the shape of an oblong.

Doubled. —Symmachus translates khoshen by δόχιον, “a receptacle” or “bag;” and if the Urim and the Thummim, being material objects, were to be put in it (Exodus 28:30), such a construction would seem to have been absolutely necessary. Hence the “doubling,” which would not have been needed merely for strength, since linen corselets, stout enough to resist the blow of a sword, were among the manufactures of Egypt, and could no doubt have been produced by the Hebrews.

A span. —The “span” was reckoned at half a cubit, or about nine inches.