Make for yourself an ark out of wood of the gopher tree. You will make compartments in the ark, and you must cover it inside and out with pitch. And this is how you will make it: three hundred cubits the lenth of the ark, fifty cubits its width, and thirty cubits its height. You will make a tso'har [roof; or, window] for the ark, and you will complete it to the extent of a cubit upward, and the entrance of the ark you will put in its side; you will make it with a lower story, a second story and a third story.
There was an official "cubit" in Egypt. In fact, there were both a shorter "cubit" (17.6 inches) and a longer "cubit" (20.65 inches). The Siloam inscription states that the Siloam tunnel was 1,200 "cubits" long. This divided by its measurement in feet (1,749) demonstrates that as late as Hezekiah's day (cf. <2 Chr. 32:4>) the "cubit" was about 17.5 inches or the shorter Egyptian cubit.
(from Vine's Expository Dictionary of Biblical Words)
Hebrew Cubit. The Hebrews, like the Egyptians and Babylonians, had two cubits, the common and apparently older cubit <Deut. 3:11; 2 Chr. 3:3>, and a cubit that was a handbreadth longer <Ezek. 40:5; 43:14>. The common Hebrew cubit was 17.72 inches and the long cubit 20.67 inches, apparently the same as the Egyptian royal cubit.
(from New Unger's Bible Dictionary)
The ordinary cubit of commerce was shorter, and has been variously estimated at between 16 and 18 or more inches, but the evidence of the Siloam inscription and of the tombs in Palestine seems to indicate 17,6 inches as the average length.
(International Standard Bible Encyclopaedia)
If the shorter cubit was used as most scholars would agree, that would make the ark 437.5 feet x 72.9 feet x 44 feet.