The average septic tank size is 1000 gallons until 1991, 1991 codes changed and houses built after 1991 are 1250 gallons or larger and new houses can have filters.
The septic design for your house should have tank size, type of system on it. The homeowner should design on a newer house, if not the town office in your town should have a copy.
Septic tank size is important. When you want to get an estimate for septic pumping.