"So make yourself an ark of cypress wood; make rooms in it and coat it with pitch inside and out. 15 This is how you are to build it: The ark is to be three hundred cubits long, fifty cubits wide and thirty cubits high. 16 Make a roof for it, leaving below the roof an opening one cubit high all around. Put a door in the side of the ark and make lower, middle and upper decks" NIV Gen 6: 14 - 16
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 ( 2 Chr. 32:4 ) the "cubit" was about 17.5 inches or the shorter Egyptian cubit.
( Vine's Expository Dictionary of Biblical Words )
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.
( 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 Encyclopedia )
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.