100% wool. NOT Superwash. White does not felt as well as other colors. On the average, stockinette stitch shrinks about 1/3 in length, only about 15% in width.
To predict how your wool will felt, make a good-size swatch, measure it carefully or draw an outline of it; then put it through the felting and drying process. Measure again and do the math to figure how much your project will shrink after felting.