Depends on how much gear you will bring and how much weight.
If you need a bear cannister for where you will go backpacking, then forsure you will need a pack that is 50-60 liters (depends on the size of your bear cannister).
Do you have a tent? Is so how much does it weigh. Get your core gear of sleeping bag, sleeping pad, tent, stove and cooking stuff, figure out how much clothing you will need. You will want a water purification system or iodine. Maybe firstaid kit. Those items will help guide you as to the size of the pack you want. A heavier pack will probably have you lean to one with better suspension.
That being said, I use the Osprey Exos 58 and it has never failed me. Great combination of volume and lightweight