The biblical dimensions give Noah's Ark a length of 300 cubits.
Genesis 6:15 specifies the ark as 300 cubits long, 50 cubits wide, and 30 cubits high. A cubit was an ancient measurement generally based on the length of a forearm, so its exact value varied by culture and period. Modern conversions commonly produce a length of roughly 135 metres, or about 450 feet, but that is an estimate.
The dimensions create a long, rectangular vessel with a length-to-width ratio of 6 to 1. Naval engineers and writers have sometimes discussed this proportion in relation to stability, though the Genesis account is a religious narrative rather than a modern ship-design manual.
The width and height are common distractors because they appear in the same verse. The requested measurement is the ark's length: 300 cubits.