How do I find out the boundaries of the parish/township I am researching?

To work out the boundaries of your parish/township, you should use the 1st edition Ordnance Survey 6" maps. These are available online - http://www.british-history.ac.uk/map.aspx?pubid=270 or at http://maps.nls.uk/os/6inch-england-and-wales/counties.html