Are you planning a cross/round country trip for this summer? Let's talk about luggage volume and have begun gathering my gear.
Let me ask you this: "How long is a piece of string?"
It really depends on what level of comfort you want when you stop for the night.
People travel with lots of stuff or minimal stuff. You probably need a heck of a lot less than you think.
Are you camping, or staying in motels? No matter what your plans, you'll be much happier taking less than taking too much. An oft repeated but good piece of advice: lay out everything you think you need and leave half of it at home.
I would recommend getting the gear you want and then finding the panniers to store it all.
At a minimum you will want a sleeping bag, hammock tent/bivy bag, a change of clothes, and spares/tools. That should easily fit in 11L panniers. Things like extra clothes, a cooking stove, and a tent big enough to sit up in is really nice to have but you won't die without them.
If I stay at hotels and go to restaurants, I'd need a bit more clothes, but could ditch a lot, so 40l will do. But I pack very light.
I traveled two months around Europe in '97 with only one pannier and a top case. Other than my riding clothes I had one pair of pants and one pair of shorts. One T-shirt and one shirt. I had one extra pair of footware and a few pairs of socks etc. Toiletries and that's about it. Wasn't camping though. Washed my clothes every few days either by hand or in someones machine.