“For the maintenance and defence of an acre’s breadth of wall sixteen hides are required. If every hide is represented by one man, then every pole [an Anglo-Saxon system of measurement equating to c.5.03m] of wall can be manned by four men. Then for the maintenance of twenty poles of wall eighty hides are required”
Correlation between expected size and actual size in measured burhs
(click on the image to enlarge)
There are two exceptions to this: Exeter and Chichester. Both of these are re-used Roman towns and it is likely that only part of the area of the town was used to create the burh.