The big ball you have on the left isn't pearl cotton. It's probably Knit-Cro-Sheen, which is a size 10. Pearl cotton has a more polished finish. Size 3 is the thickest, but very hard to find any more. Size 5 is a bit thinner, and size 8 is thinner, still and comes in a ball instead of a skein. You can use all 3 sizes of pearl cotton, or the Knit-Cro-Sheen. It all works well, because it is cotton, and will hold a knot.