Have you thoroughly read the book, it may say somewhere what they mean when they say a skein. Anchor wool is 10m but I think DMC is 8m so not all skeins are equal. As for size of canvas, 20 x20 is a standard cushion size. If you want to change from 12 hpi to 10 hpi you will end up with a larger cushion, if you follow the pattern as it is. Say the pattern is 240 st x 240 st at 12 hpi that works out at 20 inches but if you stitch that same pattern on 10 hpi you would end up with a finished sized of 24 x 24 inches.