Forgive me if I think in pack volume units of cubic inches rather than liters. My old rule of thumb is that I need roughly 1000 cubic inches of volume for each 10 pounds of load. So, if I was thinking in terms of a 35-pound load, then I would select my 3500 cubic inch pack.
For winter and due to the extra insulation bulk, I generally select my pack around 4000 cubic inches despite the weight being 28-30 pounds. For summer and due to the lack of anything really bulky (with the possible exception of a bear can), I can keep it around 2000 cubic inches for a total weight of 20 pounds or less.
For one high expedition, I had to carry loads of around 60 pounds, so I selected a big 6000 cubic inch pack. I'm glad I don't have to do that again.
Many of the REI stores have a set of sandbags located in the backpack department, and you can start from the volume and work your way toward the weight, or you can start from the weight and work your way toward the volume. The same stores sell bear canisters of a known volume, so you can ask to slip one of those into a pack that you are selecting.
I've known guys who were all fired up to keep their pack size unusually small. Then they remembered all of the extra winter stuff that they needed to carry. They ended up lashing this and that all over the outside of the pack. I guess that is acceptable, but it gets bad results when you are skiing and balance is critical. Then something pops off the lashing and is lost.
–B.G.–