The key unit of land measurement in Lancashire up until the 19th century varied considerably. The best treatment of the subject is E. H. Smith’s paper of 1958 published in the THSLC.  Page references here are to the pdf offprint of his article, available on line.  Smith discovered some ten different measures of the acre, varying in size from 4,840 sq yds to 11,900 sq yds. 
This variation has obvious implications for the historian, as Smith noted:
The size of the acre is of interest not only to the social historian in his study of the way in which people lived, or to the political historian in assessing tax values and the like, but also to the economic historian comparing and contrasting farming efficiency then and now. If he is not aware that one acre on paper may be two statute acres on the land … his findings are more than suspect. 
The variation comes about because of the different length of the pole used to measure length, the statute measure being 16ft 6in. The resulting difficulties in interpreting documents are compounded by the fact the length of the pole is rarely specified before the 18th century. As Smith notes:
The study of the acre is the study of the rod, pole or perch, for it was by this that the acre was measured. Since there is no evidence that the perch was ever measured in units different from the statute feet or yards, or that the acre ever consisted of other than 4 roods of 40 poles, the length of the rod is the decisive factor in determining the size of the principal unit of land measure. Once we have determined the length of the rod, we know the size of the acre. The difficulty which arises is that the rod, pole or perch was so much a part of the life of an area, so accepted a part, that its precise length is seldom mentioned. 
In 1697 a surveyor produced a series of tables to allow him to convert six different Lancashire measures, varying between 18 feet and 25 feet six inches, to the statute 16 feet six inches. This leads Smith to warn, ‘No attempt to estimate the incidence of population, land utilisation or fertility, or even the comparative wealth of a farmer or landowner can be successful, unless some attempt has been made to define the type of measurement in use at any particular place and time.’ 
The measures most widely used in Lancashire, according to Smith, were either 21 feet or 24 feet, with the former favoured north of the Ribble and the latter to the south.  In Preston, the 1774 plan of the town and the survey which it illustrated, produced by George Lang specify a pole of seven yards, and a de Hoghton deed of 1535 does the same, specifying a rod of 21 feet to measure land in the town: both conforming to the ‘rule’, and meaning that a de Hoghton or Lang acre is equivalent to 1.62 statute acres.  This measure was still in use into the 19th century, as witness a plan of William Atherton’s Preston estate dated 1813 which specifies a pole of seven yards. There is a copy of this plan on Flickr (https://www.flickr.com/photos/rpsmithbarney/20879909855/) which is not sourced but probably relates to the Atherton map at Lancashire Archives.  And when William Cartwright was carrying out a survey for the proposed canal basin in Preston in 1801 he specified a perch of seven yards as is shown on a copy of his plan in the map drawers at Preston’s Harris Reference Library.
There is little consistency in the measures. In Leyland, in a plan on display in the town’s museum, Thomas Addison was specifying a pole of 22 feet 6 inches in 1769. In Preston, Robert Porter was making use of a statute pole of 16½ feet in 1756.  And for the medieval period Mary Bateson, in a somewhat harsh review of William Farrer’s edition of the Cockersand cartulary, takes him to task for confusing measures of length and area. She notes that the cartulary contains references to rods of 20 feet and 24 feet relating to land north of the Ribble. 
Smith further notes that measurement could also vary according to usage, from the simple Anglo-Saxon distinction between wood acres and field acres to variations according to the work involved, with different measures for hedging, draining and digging.