The September 2013 Awake has this to say about the length of Zheng He's treasure ships:
"Historical records from the Ming dynasty say that Zheng He’s treasure ships were amazingly large—447 feet (136 m) long and 183 feet (56 m) wide. Scholars find these figures problematic and hard to verify, in that wooden sailing ships in excess of 300 feet (90 m) in length are structurally unsound.
“All indications are that exaggeration has been at work in the accounts that mention the ships’ enormous size,” says one article on the subject. “A ship of about 200-250 ft [60-75 m] would make much more sense than the 450 ft [135 m] one.” Whatever the case, in the 15th century, vessels measuring over 200 feet (60 m) in length were certainly exceptional, and Zheng He’s fleet included up to 62 of them!"
I had been lead to believe that wooden ships 450' long were rather easy to build (a 600 year old man could do it!) and ridiculously seaworthy. But maybe Zheng's big mistake was building his ships with rounded, pressure-resistant ribbed hulls. Everyone knows that perfectly square and flat-bottomed is the way to go.