First Septic-Scrub may help open to soil to better drainage. There may also me an accumulation of lint from the washer clogging the soil. No chemical or bacteria will dissovle this lint, some of it must be physically removed before any treatment. If you want to build new seepage pit, there are no "rules" about how big to build it. This depends on the ability of the soil to accept water. Generally these pits for the washer do not need to be too big, but this also depends somewhat on how often you do laundry. I would check with a septic contractor or your local soil conservation office to better determine the characteristics of the soil in your area. Your local contractor can also suggest construction methods suitable for your area. The main concerns are to dig it deep enough for good water absorption, shore up the walls to prevent collapsing, and leave some sort of access to the top of the pit to minitor and allow treatment in the future.