In 06, LOTS of folks used the Vapor Trail (as did I). It’s 3600 CI (about 59 liters). It was rarely full to the brim, except as Suge mentions, when leaving KM with 11 days of supplies.
Check one out at a store close to you - it should a fair benchmark to judge various packs against.
More generally, your pack size, in a sense, is determined by the rest of your equipment. If your kit is bulky (or heavy), you’ll need a bigger (more suitable for heavy loads) container (the pack) to haul your stuff around. Of course, the opposite is also true. You don’t say if you use synthetic insulation (bag / parka) or down - down compresses very well, synthetic, less so. This can influence the pack size you’ll ultimately find works best for you.
As far as the luxuries go - I totally agree with the above comments. Some piece, or pieces, of ‘luxury’ go a long way to keeping your morale up. On these lines I had a MP3 player, my journal and an SLR-like camera plus filters for quality pics. These items were at least 2 to 3 pounds, but well worth their weight, not only on the trail (MP3), but now, a year and 4 months after finishing (camera and journal).
BTW - a sleeping pad is a necessity, not a luxury. The ground can be real cold.
Token Civilian