I'm looking into buying my first ever serious backpack for hiking/large trips. I'm unsure of how large I should go. I currently own a 35 Liter bag, which was great for weekend trips when I was studying in Europe, but I had issues trying to do 10 day trips with it. So what is a good size? 75 Liters, 65 Liters, and 55 Liters seem to be the three most common sizes above 35 Liters. However, I've read a bunch of places that 75 Liters is too large for anything but the longest trips.
My current upcoming trip that is prompting this is the Trans-Mongolian railroad. I'm not doing the entire thing all at once on the train - I plan to stop at varying places, so I don't necessarily need to bring as much with me on the train as someone else might. My guess is I'll be doing it for 4-5 weeks. I know the weather can be a bit unpredictable this time of year, so I'll have a few extra clothing items. I'm sure I'll want to buy some things as well.
Does anyone have any experience with this? Did you do the Trans-Mongolian (or any Trans-Siberian variant) and find your bag to be too small or too large?
Should I get a 75 Liter, 65 Liter or 55 Liter bag? Thanks!