In 520 BC that was Babylon most probably

An hectare is in principle a square of 100 x 100 meters = 10,000 m².
It could be also rectangular for the same area, but not too long in the main dimension 

That area was what a ploughman with a good horse was able to plough in one day ( un jour), hence the work was called a "journal" and the ploughman doing such a job was a "journalier".

A 100 meters long furrow was possible to be controlled with the eyes to see if it was straight or not.
In English it is called a "furrow line".