It had the correct proportions, unlike the ark in the Babylonian flood story which was a cube 55 metres by 55 metres by 55 metres! …
The ark resembled a huge barge. It was built just to float – it had no sails, oars, propellers or rudder.
It consisted of 3 decks and a 0.5 metre ventilation gap just below the roof, running it′s full length.
The eight images below are shown to the same scale: (Different sized monitors may display these inaccurately, but it will give a visual indication of their sizes)
Length: 19.5 metres Beam: 8 metres
Length: 67.7 metres Beam 11 metres
Length: 144.7 metres Beam: 21.6 metres
Length: 140 metres Beam: 23 metres
Length: 135 metres Beam: 22.5 metres
Length: 164 metres Beam: 25.3 metres
Length: 269 metres Beam: 28 metres
Length: 300 metres Beam: 44 metres
Noah had probably never seen an ocean or had any knowledge of ship building, but the end result was that it had the right proportions to be very sea worthy.
It was also massive in size to hold a lot of cargo, animals and people.
But could Noah have built it?
We can be utterly amazed how the ancient civilizations built enormous and complex things:
The Pyramids, Stonehenge, cathedrals, complex cranes and other machinery, and all with tremendous accuracy.
When modern engineers come to copy some of the ancient things, their knowledge and skill is often severely stretched and often they express amazement at how it was originally built.
So early mankind were clever engineers.
Also, God gave Noah the dimensions and the plan of how to build it:
“So make yourself an ark of cypress  wood; make rooms in it and coat it with pitch inside and out. This is how you are to build it …”  … “Noah was a righteous man, blameless among the people of his time, and he walked with God.” 
And Noah probably was very rich, so he would have been able to hire a lot of men to build it.
 The ark is to be 300 cubits long, 50 cubits wide and 30 cubits high. Genesis 6:15
 Genesis 6:9
 Hebrew word unknown to us.
 Genesis 6:14-15