The Back-Rollers are standard pannier size (20L per pannier). The do have a compression strap so they can even be reduced in size if they are mostly empty. You can't really go wrong with a pair Back-Rollers, especially if you also want to use them for camping.
I have had mine for around 6 years and they still work perfectly even though I don't treat them very nice. They are also just great for picking up a parcel at the post office or buying groceries or other stuff on the way home from work.

Some tips; if the pannier is very heavy and you are off the bike, try to carry it on your back like a messenger bag. There are several ways to close the Back-Rollers. In heavy downpour or if you want them to float when fording rivers, use the official method of closing the pannier or close them as a dry back. For normal use in moderate rain and snow, the "lazy method" of just rolling the top together with the shoulder strap still attached and then just securing it with the compression strap works fine.

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Regards