"Large enough for thru-hiking?"
That is awfully difficult to answer.
About four days ago, I was leaving a trailhead by car after my backpack trip. This was fairly near the Pacific Crest Trail, so lots of thru-hikers were around. Two of them had their thumbs out for a ride to town. I felt in a good mood, so I picked up these two guys. Geez, their packs were large and heavy! One guy's pack had to weigh fifty pounds, and the other guy's pack must have gone sixty. If my pack was that big, I would have been looking to get off the trail myself.
It seems that the most successful thru-hikers have their base weight down around ten pounds, plus or minus. Then add in the consumables weight, and that will depend on the number of days that you go between resupply stops. By successful, I mean that these people go long distances without injuring themselves, and the total load is low enough that they enjoy the trip. So, you see lots of them out on the trail carrying a total load of twenty to twenty-five pounds.
Personally, I own a 52 liter backpack, and it works for me. I thought that it was perhaps one size too large, but now I am thinking that it is OK. Having that extra 5-10 liters of capacity doesn't seem to bother me at all, and sometimes it becomes handy. Some others take the wrong approach. They can get all of their stuff into 45 liters, but then they keep adding stuff to fill up the 52 liter volume. On the other hand, if you have a 52 liter backpack and you have 60 liters worth of stuff, it makes a problem. You end up with stuff hanging off the back and sides, and that tends to drop off and be lost.
For volume, I don't think in metric liters. I think in cubic inches. My rule of thumb is that every thousand cubic inches of volume equates to ten pounds of load. So, a 3000 cubic inch backpack would add up to 30 pounds.
One liter is about 61 cubic inches of volume.
The math exercise is left to the student.