According to Genesis, Noah’s Ark was 300 cubits long.
Genesis 6:15 gives the Ark’s dimensions as 300 cubits in length, 50 cubits in width, and 30 cubits in height. The length is therefore ten times the height and six times the width according to the biblical measurements. A cubit was an ancient unit based broadly on the length of a forearm, so its exact modern equivalent varies.
Depending on the cubit used, 300 cubits is often estimated at roughly 135 to 157 metres, or about 443 to 515 feet. Those conversions are estimates rather than a single value supplied by Genesis.
The question asks for the textual measurement, which avoids disputes over cubit length. It also does not duplicate the separate biblical detail about the Ark’s height.