Septic Pit Size Unknown

we bought a house that has two pipes and a pit. We are in Gilbert, AZ. The house was built in the late 60's if I remember correctly. When the septic was serviced pre-purchase, we were told it was a 1500gallon capacity. I've been looking all over to figure out what the potential diameter of the septic pit would be so we would know where not to put anything. Does anyone have a reference where to look/find this? No city/county info due to the time the septic was put in.