My down bag's stuff sack size is 8x17....
My down bag's stuff sack size is 8x17. What size dry sack should I get? Can I just put the bag in the dry sack without the stuff sack or not?
I just go straight into the dry bag. You can roll these down and "compress" your bag pretty well with them. Down is a lot easier to compress than synthetic in my opinion. Keep in mind you'll need to Roll the closer a few times to keep the contents dry (but you probably already knew that).
9 x 21in [25L] = Might be a tight fit depends on if your 8x17 leaves room to compress or is pretty tight to start with.
7 x 25in [35L] = Would probably fit pretty well.
10 x 28.5in [60L] = Would be plenty big and leave some room for other stuff.