“That the sacred Cubit was very large, appears from the Jewish Calamus or Reed, which contained but six of these Cubits; and from the antiquity of this Cubit, since Noah measured the Ark with it;” –Sir Isaac Newton 
“It is agreeable to reason to suppose, that the Jews, when they passed out of Chaldea, carried with them into Syria the Cubit which they had received from their ancestors. This is confirmed both by the dimensions of Noah’s ark preserv’d by tradition in this Cubit, and by the agreement of this Cubit with the two Cubits, which the Talmudists say were engrav’d on the sides of the city Susan during the empire of the Persians, and that one of them exceeded the sacred Cubit half a Digit, the other a whole Digit … The Roman Cubit therefore consists of 18 Unciæ, and the sacred Cubit of 25 3/5 Unciæ of the Roman Foot” –Sir Isaac Newton 
“The 25 inch cubit is found in ancient Egypt, Assyria, Persia, Syria, and probably in Greece, varying from 25.1 to 25.4. In modern Persia, Arabia, Greece, Candia, Algiers, and Italy, a pic or braccio of the same length is found, varying from 25.0 to 25.3. The possibility of this widespread unit having some connection with the Chinese foot (the double of which is 25.18 +/- .04) and with the North American mound builders’ foot (1/2 of 25.20 +/- .04) should not be disregarded; though farther evidence, beyond these very close resemblances, is needed to prove a connection. Don Quiepo also connects with it the Japanese inc 75.21–i.e., 3 x 25.07. … The Egyptian form of this cubit is probably nearest to the original, as being the oldest that we have, and this gives 25.10. This is well known as the sacred Hebrew, Royal Persian, and Chaldean cubit, mentioned by Newton, Golius, Kelly, Quiepo and Oppert.” –Sir William Matthew Flinders Petrie 
Answers in Genesis is building an ark. A full size one! See this link. I think this is a fantastic project and will be a tremendous educational tool. When I heard about the project, I checked to see what size cubit they were planning to use and it appears that the current plan is to use what some refer to as the Royal Egyptian cubit of around 20.7 inches. Make no mistake, AiG’s ark will be impressive at ~500 ft long if they use this cubit. But what if the cubit Noah used was really ~25 inches as the opinion of Sir Isaac Newton appears to be? Wow. That would make AiG’s ark 625 feet long! Even more impressive!
What is the evidence? Well, first we have Sir Isaac Newton’s research in which he makes the statements listed above. He lists 6 evidences from ancient literature supporting his belief that the sacred cubit was close to 25 inches long. See footnotes for an online copy of his full dissertation. Then we have one of the 19th century’s leading archaeologists, Sir William Matthew Flinders Petrie who confirmed Newton’s idea quite decisively from actual measurements of monuments of the ancient world. See footnotes for his book which is also available online. Once you start looking, you find lots of sources which confirm the existence of this ~25 inch cubit. Mr. H.W. Chisholm, Warden of the Standards of Great Britain in 1877 gives the length of the Judean Cubit of the Sanctuary as 25.50 inches and the cubit of Chaldea, Persia, etc. as 25.20 inches  Chisholm cites Brandis and Queipo as his most important sources for ancient standards  and cites Dr. Brandis’ opinion that the Chaldean system was the original from which other nations derived their systems.  Old online copies of Encyclopedia Britannica have info about this cubit also. Strangely, many modern sources do NOT have information about the 25 inch sacred cubit, but interestingly, World Book does in their entry on the “cubit.”
So … I think Noah may have used a ~25 inch cubit. Hopefully, AiG will at least consider this evidence before building.
Did you know that originally the French specified the length of the meter as one 10 millionth part of a quarter meridian circumference of the earth? “So what?” you ask. Well … it just so happens that if we do the same thing to the polar radius (which is constant unlike the the meridian circumference ), that is, you divide the polar radius by 10 million, guess what you get? 25.027 inches! Cool huh? Very very close to the length of the ancient sacred cubit as best as we have been able to determine it. I wonder … did whoever came up with the sacred cubit know the polar radius? Hmm … interesting thought. I’m researching that topic right now. I’ll let you know what I find out.
To me, this topic of ancient knowledge is related to Biblical Authority. The Bible presents a picture of God imparting knowledge to early man directly, which stands in contrast to the Evolutionary scenario of humanity gradually increasing in knowledge with no input from God. Also, if the Ancients already knew Copernican astronomy 5000 years ago (as Newton believed they did) then what else did they know? Maybe we could learn a thing or two by studying them more.
MISCONCEPTIONS ABOUT THE SACRED CUBIT
Misconception #1: Petrie repudiated the ~25 inch cubit. Some modern writers aren’t aware of this cubit, perhaps because of the confusion generated amid the many pyramid theories of the nineteenth century. Some people think that when Petrie repudiated Smyth’s pyramid theory of the base circuit representing the solar year that he repudiated the ~25 inch sacred cubit also. I suspect that this idea is due to various sources only partially quoting Petrie, quoting only this snippet “and it is the more unlikely as there is no authentic example, that will bear examination, of the use or existence of any such measure as a ‘Pyramid inch, or of a cubit of 25.025 British inches” but failing to read the context and failing to read further on the same page. A complete reading gives
150. The theories of the widths and heights of the passages are all connected, as the passages are all of the same section, or multiples of that. The entrance passage height has had a curiously complex theory attached to it supposing that the vertical and perpendicular heights are added together, their sum is 100 so-called “Pyramid inches”. This at the angle of 26º 31′ would require a perpendicular height of 47.27, the actual height being 47.24 ± .02. But in considering any theory of the height of this passage, it can not be separated from the similar passages, or from the most accurately wrought of all such heights, the course height of the King’s Chamber. The passages vary from 46.2 to 48.6, and the mean course height is 47.040 ± .013. So although this theory agrees with one of the passages, it is evidently not the origin of this frequently recurring height; and it is the more unlikely as there is no authentic example [in the Great Pyramid], that will bear examination, of the use or existence of any such measure as a “Pyramid inch,” or of a cubit of 25.025 British inches.…
1. There is doubtless a well-known ancient cubit of 25.3 inches; but that is decidedly not as short as 25.0, nor is it found employed in the Great Pyramid. [Emphasis and bracketed text mine. See this link for Petrie's full "Pyramids and Temples of Gizeh"]
So Petrie was merely repudiating the idea that the sacred ~25 inch cubit or pyramid inch could be found in the Great Pyramid. He was not repudiating the many examples of the ~25 inch cubit found in many parts of the ancient world by the great archaeologists of the 19th century. It’s a little odd that he says it’s decidedly not as short as 25.0 because the examples he found in Jerusalem did in fact average 25.00 +/-0.03 inches. (Inductive Metrology, p. 75). I suppose he means the average worldwide is not as short as 25.0.
Misconception #2: There’s nothing special about the 25 inch sacred cubit – there are many lengths of the sacred cubit. It is true that the Jews used shorter cubits but these cubits were not called ‘sacred’ or ‘cubit of the sanctuary.’ This terminology is only applied to the ~25 inch cubit.
Misconception #3: Nineteenth century archaeologists discovered a “well known” sacred cubit, but 20th century archaeologists repudiated this cubit. I heard this claim recently so I challenged the person to give me an example of a 20th century archaeologist who repudiated the ~25 inch cubit. He couldn’t give me one.
 Newton, Sir Isaac, “A Dissertation upon the Sacred Cubit of the Jews”, Source: Miscellaneous Works of Mr. John Greaves, Professor of Astronomy in the University of Oxford, Vol. 2, pp. 405-433 (London: 1737). Link to full text online. Search the page via CTRL-F for “Noah”.
 Newton, op cit, Search the page via CTRL-F for “Noah”. Note – 25 3/5 unciae equates to 24.83 inches if we take a Roman foot as equal to 0.97 English feet.
 Petrie, Sir William Matthew Flinders, “Inductive metrology: or, the recovery of ancient measures from the monuments”, (London: Hargrove Suanders, 1877), pp. 133-134. Link to full text online.
 Chisholm, Henry William, Warden of the Standards for Great Britain, “On the science of weighing and measuring and standards of measure and weight”, (London: Macmillan, 1877), p. 46. Link to full text online.
 Chisholm, op cit, p. 24.
 Chisholm, op cit, p. 34.
 Depending on where you take your meridian circumference (which passes through the poles) you will get a different number … but there’s only one polar radius
If you have any more sources, or if you have any further evidence that Noah used a 25 inch cubit, please comment.