Septic tank size is based on the flows for the building. The prefabricated concrete suppliers in your area can also make suggestions. As recommended above you will need to mound the soakaway pit, commercial suppliers also sell plastic systems to increase drainage areas. As a drainage media typically well graded sand and gravel are used. I've seen septic designers just drop off available sand to get the estimated permeability from grain size to use as a design permeability.