I'm not touring year round, but, if anything, I've found that the shorter the trip, the more gear I'm willing to carry. I like to think that if I were to go on an extended tour, I could pare down the gear a little more. Although the wild card in that is "year round" likely means doing some cold weather touring, and that always involves more gear.
I started out with 4, large (approx. 20 liter) panniers. Scaled my gear down to two, 20 liter Ortliebs (with other, random gear stashed other places on the bike). But I felt like I could get a more balanced load with 4 smaller panniers, rather than the 2 20 liter set. But, I ended up switching my front end to more of a bike-packing set up. Currently using two Front-Roller/Sports-Roller panniers on the back (12.5 liters each), a handlebar roll (15 liters), and a variety of other options should I need more storage, such as cargo cages on my forks, a frame bag, and the top of my rear rack. If I were to go back to a strictly pannier set-up, I'd go with 4 of the 12.5 panniers. If I were worried about having enough clothing/bedding for cold weather, I could see subbing in 2 20 liter bags. Having already gone away from having 4, 20 liter panniers, I have no desire to take 35 liter panniers.