I ordered 25lbs and got 3 large pieces... each one was 10 - 12 inches long. One was a large boulder shape... the other 2 were more irregular. Based on the those sizes, you likely won't need more that 20 - 25lbs with maybe a little extra for playing with the aquascape. If you want smaller pieces, make sure to put that in the order notes. When I ordered 20lbs of tonga shelf, they sent me one giant piece that was 20in at the widest point. It seems they like to send larger pieces, which may or may not be what you want.