After listing the series of forts Version A of the Burghal Hidage includes a note describing how:

“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)

It can be seen that there is a close correlation between the expected size of the burh (taken from the formula given in the Burghal Hidage) and the actual size in cases where it has been possible to locate and measure the extent of the burghal defences.

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.