Typically, the dimensions of fiberglass septic tanks range from small units with a capacity of around 500 to 1,000 gallons for single-family homes to larger tanks exceeding 2,500 gallons for multi-family dwellings or commercial establishments. The standard dimensions for a 1,000-gallon fiberglass tank are approximately 8 feet in length and 4 feet in diameter. These dimensions can vary based on the manufacturer, but maintaining a consistent capacity ensures proper waste processing.