I highly recommend making your own stuff sack, tailored for your bag and for your pack. I like the bag to be right up against the lumbar of my back, filling the bottom of the pack. If a stuff sack is to be used in this fashion (as opposed to a trash bag, another good option), it must be custom made to the bottom dimmensions of your pack.
I make my stuff sacks about 50% larger than the one supplied by the maker, and have not had a reason to go smaller. No point stuffing it more than needed.