poopinleachline, how many bedrooms do you have in your house? There is a way to estimate the needed size based on number of bedrooms in the house.
You will need to contact your local department of health and sanitation and have them confirm how large of a tank you need. Keep in mind that the size/output of the tank also affects the size of the leach field, although that is also determined by the rate of soil percolation. If your leach field has become saturated it will need to be cleaned out.
My soil percolates fairly well but for a one bedroom house we had a 500 gallon tank. We had to upgrade it to a 1,000 gallon tank because 2 more bedrooms were added. The general idea is that the # of bedrooms is indicative of the number of occupants and is used as a generalization of how much water will be used.
There is a thread on the process of getting a new septic tank here: http://www.plumbingforums.com/forum/...e-system-5653/
Steps may vary for you depending on your location.