I used to tour with 44ltr Lone Peak panniers. That was the standard size for a lot of companies. Then, I found myself living on my bike. The first pair of panniers I bought off Amazon for cheap. Didn't expect them to hold up more than a year, but they did for 2.5 years until they were stolen (along with the bike. Or the bike was stolen along with the panniers. I'm not sure what the D had in mind.) They were replaced by a stranger by Axiom's 55ltr panniers. VERY large for me, but then soon after I started nomading around the west, so it worked out. For those of us who live on our bikes, larger than normal panniers (which are difficult to find, especially used) are quite helpful.

As a side note, I realized last year that large panniers are like buying a house: Every time you size up, you acquire more because YOU CAN. This is how everyone in this country has ended up with a storage pod, a behemoth RV, etc.