It all depends how much gear you take with you. That is your starting point.
I used to ride with Ortleib back rollers. They are massive and I would make sure I filled them because I had the space. Then I reduced the kit and only used front rollers on the back. Much better. Then I reduced the kit even more. I now have a Restrap holster with a drybag on the back and a home made holster on the front with a drybag. I can travel indefinately with that.
If you are carrying cooking and camping kit you obviusly need more storage space. But luggage on a bike are like cupboards at home. The bigger they are. The more stuff you will find to put in them. If you dont have the space, you need to be more selective.
I would agree - the bigger the bags the more you take -
This is my set up including tent, sleeping bag, and mat, stove, food for at least three days , electricals. book, spare clothes, including all waterproofs, ( this is Scotland after all) and a rudimentry repar kit and so forth... There's quite a lot of laundry hanging off the back there - mobile clothes line type effect !!
Total weight inc bags not in excess of 12 kg
View attachment 443356
Upping the ante now
A glamorous woman standing next to my bike - in front of some water
It also means you can load the whole lot easily onto a train - and even hang it up fully clothed in one of them dinky bike wardrobes as in Virgin trains.
Anything that really needs to stay dry goes in a dry bag - even though the back ones are allegedly waterproof - you've still got to get the stuff to the tent - and i prefer to keep the bags on the bike .
I think I'm ready to replace the front panniers - mainly due to their age - but they don't need to be any larger.
I've not yet had the luck to get away for more than about 18 days at a time yet - but i see no reason why the same luggage wouldn't do me for much longer trips .
