When buying potatoes, look for those that are firm, well shaped and free of blemishes. Avoid potatoes that are wrinkled, cracked or sprouting. If kept in a cool, dark, well-ventilated place, most potatoes will keep for up to 2 weeks. However, new potatoes should be used within 4 days of purchase. Generally, three medium russet potatoes or eight to 10 small new white potatoes equal one pound. One pound of russet potatoes equals approximately 3-1/2 cups chopped or 2 to 3 cups mashed.