Rashi on Exodus
זרת ארכו וזרת רחבו A SPAN IN LENGTH AND A SPAN IN WIDTH, being double (Exodus 39:9) and it lay on him in front over his heart, as it is said, (v. 30) “And they shall be upon Aaron’s heart.” It was suspended from the shoulder-pieces of the ephod which came from behind him upon his shoulders and hung over and fell down a little way in front of him, and the breast-plate was suspended from them by means of the chains and rings, as is explained in the section (vv. 22—29).
Rashbam on Exodus
כפול, something like a pocket. This was necessary in order to place the urim vetumim inside it.
Rav Hirsch on Torah
V. 16. רבוע יהיה כפול: es war zwei Spannen lang und eine Spanne breit, so dass es zusammengeschlagen ein Quadrat von einer Spanne bildete.