The Volume of Panniers: How do you decide the volume needed?

Do you have any tips or feedback regarding the volume of bags needed? Context: As part of a month-long bike journey, a year later, I'd like to purchase some bike bags. I would go camping with a tent, sleeping bag and mattress, kitchen utensils, and clothes. I hesitate between the Ortlieb back roller plus 2x20L either 40L or 2x35L or 70L. However, I fear 40L is too little and 70L is too bulky. Do you have any experience with the pair of 70-litre bags?

Better to have space in a bag and not need it than needing space in a bag and not having it.
but it is also true that the more space you have, the less careful you are when choosing what to take and you end up taking more things which translates into more weight
that is called discipline. And in most cases the "taking more than you need" problem is resolved during a test ride, where the additional weight will come into play. Then there will be a lot of free space again. If it can't be done discipline, then lesson-learned will.
correct, I learned over time, today I have the same paper checklist as 10 years ago, many items are crossed out
2x20L is too little for me, especially for a year trip, but I will avoid 2x35L because all the weight would be on the back of the bike. In my opinion, the best combination would be 2x20L in the back and 2x12.5L in the front (if you choose Ortlieb bags)
Hello  actually no matter the back bags I would also put some in front (weight distribution) Therefore the question is rather; do I opt for 70+25=95 or 40+25=65 I would also put no matter my choice a handlebar bag as well.
lost, 40+25 and I'm going! Then you can add a bag to your bags (for like tent and sleeping bag).
To be more precise, before I had to store my kitchen/tools for the bike, and what to eat for 4-5 days. And, a bag for mainly the fringes, and the other for sleeping and a few other things
it depends on how much you want to be charged I prefer not to take oversized bags because I tend to overfill them otherwise. Otherwise, I am finishing a year-long journey and I have exactly what Joel is proposing. Sounds perfect to me. It distributes the weight of the bags and not only pulls your luggage but also pushes them. In particular, it prevents back pain.
I don't know my volume in litres but I have 2 panniers each about 5kg. One is for tent and sleeping gear +. The other is mainly for clothing etc. I also have a top bag which weighs around 4kgs. This contains a cooking stove and food. My front roll bag will have my phone and waterproof clothing
I try to get as lightweight as possible. I learned to take on a 1-year long trip as much as I would take for three days. However, I am freezing very quickly, though I take a couple of long sleeves, thin woollen jumpers and 2 pairs of long, thin woollen underwear (trousers). Keep in mind, that you always find a place to wash your clothes. I have two VAUDE environment-friendly aqua back panniers - with two smaller outer pockets. They have 25 l each, a Silkroad bag (on the rear rack), but only because of my medicines and because I am a woman. I also will have a handlebar bag. Last year I had a 30 l big bag on the rack for my winter coat, which was too clumsy for me - maybe it would be suitable for the tent, mattress etc. 1 pic = last year, 2 pics = this year
The bigger the bag the more you sail. A small bag forces you to grab efficiency.
Preferably you should travel lightweight. When you are out in the wilderness, the frugal living style has its own taste. If you wish to live lavish then such adventure has no charm. Avoid carrying too much of stuff, however, no compromise on your clothing etc, as you sweat our a lot, evening you wish to relax after a shower and a good dinner. Bon voyage.
70 is way too big. I have two 40 and two 20 with a rack pack for my tent and sleeping gear.
The smaller the better. The bigger the bag the more stuff you will take that you don't need