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