My old house has a Steel Tank from the 1950's and it was only a 500 Gal tank with four people on it and we never had a Problem, My EX has the house now and the leech field is starting to act up, she is getting the house added to the Town Sewer system, She is a ways out of town but they are running it since they have others also hooking up to it.
I now have a 1,250 Gal Cement tank it seems to do fine, I did have to add another 80 Ft of leech field though, the whole system is about 5 Years old. I have also have a little grey water thing going.