Customary furlongs derive from the length of the furrow ploughed by a team of oxen before resting and turning the plough. As its Latin name indicates, the furlong was notionally 40 perches - 220 yards - in length. In reality, the size of the furlong would have been affected by the nature of the soil, the shape of the field, and other contingent factors. It is not even certain that the Domesday furlong was always a measure of length; several Domesday statements imply that it was a measure of area.
In circuit 2, in the counties of Cornwall, Devon and Somerset, furlongs are recorded as both a unit of assessment and a peasant landholding units. These furlongs were a quarter of a virgate, and therefore nominally 7 1/2 fiscal acres. There are, however, indications that the Cornish hide was only 40 acres, making these furlongs only 2 1/2 acres. These, of course, were fiscal acres which could bear a variable relationship to the customary acres on the ground. Like other Domesday measurements, the furlongs in circuit 2 could vary in size from area to area, or even within the same manor.
Despite variations in size between furlongs, however, individual furlongs could, like other peasant holdings, remain fixed in size over generations, even centuries, their integrity maintained by the power of the lord or the customs of the manor. They provided the stable base for both the manorial and the assessment systems.
For more detail on the customary furlong, see Frederic Seebohm, The English village community (fourth edition, 1915); and for the furlong in circuit 2, see the note to DEV 1,4 in Domesday Book, vol. 9: Devon, edited by Caroline and Frank Thorn (2 vols., Phillimore, 1985), which can also be found online at: https://hydra.hull.ac.uk/resources/hull:535..