Hello there @PRN !
Thanks for reaching out to the Community! I typically recommend getting the same size compression sack in liters as the stuff sack that comes with the bag. This way you wont have a fight getting your bag into it, but you can still cinch it down quite a bit.
If you have the Anvil 30, it comes with a stuff sack that is just over 14 liters in capacity size, so I would plan on getting a 14 liter compression sack. Here is a popular option .
Hope this helps!
Hi @PRN and great question. @REI-AlyS is right -- 14 liters is hands down the safest bet. You could possibly go as low as a 10 liter with a 650-down bag, but the long length might make the fit a little tougher.
Also, depending on where you are using your pack, there are also waterproof options for compression sacks as well. If you don't need it to absolutely stay dry, then don't spend the extra money or add the extra weight. Here is an example of a waterproof compression sack.
Hope this helps, and have fun out there!
@PRN, here's another perspective. This was recommended or shown to me, I tried it and now will not go back (until I do, lol)
Forget the compression bag or any stuff sack.
Line your pack with something like a trash can liner, really any large waterproof bag that is as large as the inside of your pack (btw keeping all your stuff drier)
Then STUFF your bag into the bottom of you pack (inside the water proof liner).
This has the tremendous advantage of the bag filling out all the bottom corners of the pack, no more wasted space! And, as you pack all your other stuff into the pack, the bag is going to be compressed.
A compression stuff sack turns your bag into a small bowling ball and creates a lot of odd space around it that's hard to fill. Plus you have the weight of the stuff sack as well.
It took me several years to get past the idea of 'all my other stuff' pressing down on my bag, plus knowing my DRY bag is my last line of defense against hypothermia.
But, now I do it. I make sure everything in my pack is going to be dry (with the liner), and everything else is in some sort of bag or ziplock, so my stuff is always as clean as expected while backpacking.
Just my two-cents.