Just some stuff to think about...
Are you kitting out for an overnighter? A week? A month? A year? Indefinitely? Is your riding gear waterproof, or will you be packing a rain suit? Shoes for off the bike? Clothes for off the bike? Clothes for on the bike? Which leads to...
Where are you going? What season(s) is it/will you be riding in? Whats the weather like where you are, where you are going, and along the way?
Are you going to be camping? If camping, are you bringing your home to the woods or are you a minimalist that basically needs a level place to lay down?
Do you need to or want to bring a laptop? Tablet/iPad? GPS, hardwired or battery powered? Paper map backups? Camera(s) and associated accesories (lenses, tripod(s), cases, etc)?
Spare parts and maintenance items... do you feel that you have to ride out better equipped than most dealership service departments, or do you carry basically nothing beyond whats needed to do an oil change if that? What tools beyond the supplied kit that comes with stock bike? Tubes? Plug(s)? Oil filters? Air filters? Chain+sprockets? Final drives and fuel pumps? Are you going to carry extra gas? How much?
Are you buying food in town all the time, or are you getting off the beaten path and eating and cooking in the woods? How much water are you going to be carrying? How much
Extra stuff beyond the riding itself... do you have a hobby or a job that requires special equipment on the road? There's a guy on ADVRider who carried a viola with him. I've personally had a surf board rack for my bike for a while and have seen others with one.
Again, just some stuff to think about. My summer overnight or weekend camping kit took up about 35L (Kriega US-20 tail bag, original 8L each Wolfman Enduro saddlebags) not counting my Wolfman Enduro tank bag. I did a 3 week, 6600 mile dirt road and back road trip in August in the US with 38.5L (Dirtbagz Scout panniers, Wolfman Enduro duffel) not including having my air mattress strapped to the back seat in front of the duffel.
A trip last spring, also 3 weeks and 6600 miles, required over 50L of storage (wolfman enduro saddle bags and duffel, 20L waterproof stuff sack for clothes) and strapping my tent to the outside of my bags on top of it. On my one way ride/move to Alaska this spring, my winter rated down sleeping bag took up nearly 20L of space by itself... I was probably around 60L of storage but I was moving, not just riding, so I carried more than I normally would and had to make room for my DSLR camera.
If I hadn't camped and cooked at all, I probably would have just taken a small tool roll and a waterproof duffel, probably a Kriega US20 or 30, just for layering and convenience of not doing laundry daily or every other day. In summer, a US10 would have worked for those 3 week trips easily.
cliff notes: It varies.