The word acre derives from the Latin ager, meaning a field. Field - or customary - acres must be distinguished from the fiscal acres of the assessment system, these being normally 1/120th of the fiscal hide, though fiscal hides of 40, 48, and 60 have been identified in some counties, and in others no consistent relationship between hides and acres can be established. Fiscal acres had a variable relationship with customary acres, which themselves could vary widely in extent. The Domesday acre was not an exactly measured area. As Maitland observed:
to tell a man that one of [his] acre-strips was not an acre because it was too small would at one time have been like telling him that his foot was no foot because it fell short of twelve inches
Since customary acres were normally composed of scattered furlong strips whose size varied according to the nature of the soils and the shape of the fields, standard units and exact measurement is not to be expected. Even in the nineteenth century, customary acres varied in size from area to area and county to county, some acres being four times the size of others. Notionally, however, the customary acre was 40 perches long and 4 wide, the width of 4 cricket pitches by a furlong in length.
For more detail, see Frederic Seebohm, The English village community (fourth edition, 1915); J.H. Round, Feudal England (1895); F.W. Maitland, Domesday Book and beyond (1897); James Tait, 'Large hides and small hides', English Historical Review, vol. 17 (1902), pages 280-82; R.R. Darlington, 'Introduction to the Wiltshire Domesday', Victoria History of the county of Wiltshire, vol. 2, edited by R.B. Pugh and E. Crittall (1955), pages 42-177; and 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..