300 cubits long
50 cubits wide
30 cubits high

A Cubit is the lenght from your elbow to the tip of your middle finger - usually about 18 inches. Of course if Noah was very big, his cubit could have been 2 feet or even more!
Genesis 6:15
And this is how you shall make it: The length of the ark shall be three hundred cubits, its width fifty cubits, and its height thirty cubits.