This link may answer your questions - http://www.reddit.com/r/AskHistorians/comments/11rigy/in_class_today_my_professor_mentioned_the_chinese/

In brief it may have been possible to create such a vessel only if it essentially never moved and was situated in a very gentle river/lake such as Caligula's Giant Ship.

No wooden ship even close to that size could handle anything more then the lightest conditions, certainly a ship that size would tear itself apart almost instantly if it did. The ships used in the fleets would have to have been more reasonably sized to actually make the travels they do simply based on the physics of the universe and the material strength and properties of wood.

Even with all the technology and materials available in 1909 the Wyoming was only 350ft long of actual ship and that had significant issues when out at sea, if we can't build it now it would be impossible for the Chinese to have produced a ship as large as claimed that actually sailed with the Treasure Fleets at that time in History.

This National Geographic article states that all of the ships in Columbus's fleet could be stored "on a single deck of a single ship" in Zheng He's fleet.

The article itself doesn't list any sources that I can see however.

In the book When China Ruled the Seas on page 80, author Louis Levathes argues against the original description that Zheng He's ship was 460 foot long, instead estimating it at something more like 400 foot long and 160 wide instead.

So there's the conservative estimate on the size of his ship. A quick glance at Wikipedia shows the lengths of the Niña, Pinta, and Santa Maria to be 50, 56, and 62 foot, respectively. That's around 168 feet total.

So all three ships strung bow to stern are less than half as large as Zheng He's ship, according to Levathe's argument. In that case the model should actually be much (around 2 times) larger to show the proper scale.

In the comments of the original post, someone mentioned that it was unknown if Zheng He ever actually sailed the open ocean, and that this was most likely a boat for parading wealth/power in rivers and such. However, the way I learned of Zheng He was that he was one of the few who did travel the world from China, and was burned along with his fleet/crew/goods on his return. My question is, which of these is correct (if either)? What exactly did Zheng He actually do?