I have a Mountain Hardwear 20°F Piute...
I have a Mountain Hardwear 20°F Piute Sleeping Bag - 650 Fill Power Down, Mummy for Women.
Would like to get it as small as possible. Which size sack should I get?
measure the dimensions of your stuff sack and go with the corresponding size of compression dry sack.
Ryan's method is spot on. Your bag stuffs at 8x16" per the specs. You can make it work with a small STS comp sack or go up to a medium for a slightly bigger (1") diameter and easier time of stuffing it, but still with the same amount of overall compression in the length. I personally prefer a bigger size when using a roll-top sack because it just makes it easier to get the required # of flips on the top to seal it, and because I like to let the straps do most of the work for me.