Mick has summed it up pretty well, square area of roof, paved areas serviced by surface drains, it will also govern how deep you go and how much square area around the absorbtion trench you need, which will give you cubic metres of drainage material ( blue metal etc.) you need to drain storm water away from your house.
Have a look at AS3500.3.2 1998
or Im sure the HB39 (national roof plumbing and wall cladding code) may give you some answers.....
PS, check with local council, who is usually the authority that governs storm water.
Plumbers were around long before Jesus was a carpenter