The capacity of your battery bank relates to how 'long' you wish/need to operate your 'house' (some list of equipment) when not connected to shore power, solar is not available, and without running a generator.
Assuming you have performed an energy audit/budget (http://jdfinley.com/energy-audit-watt/
), the remainder of the calculation is pretty simple. Of course, that ignores the fact that it is incredibly hard to know whether two days is needed or five days (or x days)!!