Whey protein is the protein extracted from dairy products. It is pretty much the best way to naturally add size. It can be found in any vitamin store like GNC or vitamin shoppe.
If you're looking to add size, consume calories. lean meats, fish, chicken, vegetables, fruits, and carbohydrates. You need to find your daily caloric intake, and eat more than that. Pretty simple. Don't just rely on suppliments and shakes to get you there. Their all pretty much junk you don't want to consume.