First thing to say here is don't call it a backpack. As military types and any experienced adventurer will know you call it a bergen - the term was coined by British soldiers and comes from the original '42 Pattern external 'A' frame rucksack that was closely modelled on skiers' rucksacks made or used at Bergen, Norway. Essentially they had to be large and tough enough to carry equipment that would support any long range, independent operations.
My experience is that you want a small one, because whatever size you get you'll inevitably end up filling it – what you can lift in the gym won't matter on a four day gruelling trek. Along with the size, pack light, and make sure to get a waterproof liner or drybag for your bergen too. The last thing you want is rain to cut your adventure your short. Everything you must keep dry double-bag with smaller individual drybags because that way if your main liner fails your stuff will be safe. It also means you won't get rain on your kit when you open the thing up. Use different coloured drybags too to help you find what you're looking for.
This may sound like a hassle but when out in the field it makes such a difference. Do pay attention to where everything is and repack things in the same place, with things you'll regularly need (torch, map, snacks) near the top. Store heavier items at the bottom of the bergen for comfort, with an even distribution of weight between left and right, and try not to pack hard edges or corners against where your back will be or they'll dig in.
Finally do try on a selection of bergens before settling on one. Some come in different spinal lengths and will be more comfortable than others. You can carry double the weight in comfort with a bergen that fits compared with one that doesn't.