Sleeping bags often have a recommended stuff sack size given either in dimensions or volume i.e. 6 x 11 inches or 5 liters.
Is the recommended size for how small it can get for backpacking or is it more the size of what one could store the bag in?
To put it another way, does the recommended stuff sack size fall on the more or less stuffed end of the spectrum?
I owned multiple (down) sleeping bags that came with their own specific compression bags. I assume these compression bags are chosen by the manufacturer to 'fit' the sleeping bags in question (as to how they might evaluate this I have no idea - I would assume it's mostly just a question of how easy it is to stuff them in there).
When putting the sleeping bag in these compression bags, and after compressing them, the sleeping bag is pretty much as small as you can feasibly get it just by using your hands and some attached straps. If I would sit on it I might maybe compress it some 10% more, but that is a hassle and not worth the little space saving for me.
Bottom line: on the sleeping bags that I've used (and owned) the original manufacturer-supplied compression bags were on the rather small end, i.e. high compression, of the spectrum.
The recommendation is mostly so people buy the correct aftermarket stuff sack size. If I go to buy a stuff sack (we have, me and my partner, a combined 7 or 8 sleeping bags), I actually test the stuffing myself.
You can follow the recommended volume and try to buy a stuff sack with the corresponding value but I found that the listed stuff sack volume is usually uncompressed, so it is its maximum (I have the Outdoor Research models in mind). If your sleeping bag has an advertized stuffed volume, this makes things slightly easier but if you look at MEC's specs for sleeping bags, here a -12°C Western Mountaineering model, they recommend a 20L compression sack for a 8.1L compressed bag. That's most probably too large a size.
For down models, I usually try to aim for somewhere between 1.5 to twice the volume. No more. Synthetics needs a bit more volume for the initial stuffing before compression. As long as it is not overly hard to bunch the sleeping bag in the uncompressed stuff sack, it will work just fine and you'll get the best compression possible. With a too large stuff sack, you'll bottom out the straps before you're done compressing. For example, I have 10L and 15L OR stuff sacks where one of my sleeping bags fits perfectly in the former while I can't completely compress it in the latter. The bag probably compresses to 5-6L.
I try to squeeze until I can't possibly cinch the straps anymore, even using my full weight. This works fine with my oldschool, heavy, and bombproof stuff sacks but with the ultralight models that are most common now, I'm always in fear of ripping it apart, which I've seen happen countless times when I worked retail.
You stuff it into a bag that fits it reasonably. To small and it's a lot of work to stuff it. To large, and you don't have room for your other stuff.
I prefer to use a bag that easily fits my sleeping bag. Then the remainder of the volume carries spare socks, fleece, rain jacket. (I still use an external frame pack, so the stuff stack is readily accessible.
Note: Do NOT store your sleeping bag in it's stuff sack. Repurpose a box, or a pillowcase or old hockey back. You want the bag to have close to it's full loft when it's hibernating between trips.
