Like many Domesday measures, there is no certainty about the extent of the customary rod, or even whether it was a linear or areal unit, though it is most commonly used in Domesday as one of the dimensions of an area.
As a measure of length, it would have been notionally 5.5 yards; of area, a virgate or quarter of an acre. It is not always possible to distinguish the rod (virga) from the virgate (virgata) as the scribe might use the same abbreviation (virg', or even simply v') for either. Both had the same root meaning: rods were quarters of acres; virgates quarters of hides.
For more detail, see Frederic Seebohm, The English village community (fourth edition, 1915); F.W. Maitland, Domesday Book and beyond (1897); Philip Grierson, English linear measures: an essay in origins (1972); and Philip Grierson, 'Weights and measures', Domesday Book: studies, edited by Ann Williams and R.W.H. Erskine (1987), pages 80-85.