Wow you guys impress the pants out of me! I'm doing 6 weeks in the USA with a 50L pack (not stuffed full as it's only allowed to be 10kgs to meet carry on restrictions). This is about right for me... it's more about weight than pack size for me, as it can be helpful to have a bit of air in our pack it makes it easier to retrieve stuff and also means I can put my shoulder bag inside the pack if needed. The main modification I had to make was to pick really light weight shoes. I live in my doc martens, but we're going to Vegas where it will be hot, so I had to get some lightweight sneakers (my skate shoes were something like 1.4 kilos and the sneakers I got were more like 400grams). I'm also taking some dressy flats (240grms) and some flip flops (170grms). So yep, it can totally be done, even for someone who thinks they can't "live" without 4 pairs of shoes!
The rationale for the dressy shoes was I can use them to dress up some of the outfits if need be. My travel clothes are focused on Smart Casual made from sensible lightweight fabrics (for example Jeggings rather than jeans, as I could hike in the jeggings comfortably, but not jeans and because they are a dark colour they can be dressed up too. My thermals are classic black made from quality merino wool, with my black and white checked mini it's good to go for a night on the town but equally helpful when camping.) The main challenge was the variety of temperatures and the fact we are going camping (includes incredibly basic camping gear - i.e. a sleep sack and insulated cooler bag for our night in death valley where we're gonna rough camp in/near the 4x4 we are hiring.) The rest of the time we are using a campervan which comes with mattress, cooker etc. We're also visiting Yosemite and San Fransisco in the same trip so the full temperature range is being covered (well not snow, but for Australian's this is a full year of temperature swings!)
I've also got a bag full of technology (tablet computer, camera, phone etc) and travel medicines.
We're budgeting $12,000 total for this trip (2 people, 6 weeks, including flights from Perth). Accommodation is a mix of national parks camping, staying with friends, 3*-4* hotel, air b-n-b and hostel (private rooms).