Rucksack manufacturers do a great job of giving the impression that their designs have been honed over decades, have undergone field tests and wind tunnel tests, and as ergonomic as an lyrca vest. The reality is, it’s all about size.
For an overnighter you’re probably looking at a 60L to get everything in to. For a day trip (which isn’t wild camping, strickly speaking) you can get away with 45L.
The length is important as well. To fit a rucksack properly you need to look at the distance between the waist strap and the shoulder straps. Ideally, most of the weight is carried by a good fit at the waist. If you get this, then the weight is carried as close to your own centre of gravity as possible, and won’t swing about and pull you off balance. Once you’ve got this, then the shoulder straps can be used just to pull in the top of the rucksack close to your centre line. There really shouldn’t be that much weight going through those straps.
If you can get this waist to shoulder distance fitted properly, you’ll be able to heft weights you thought unimaginable, with disturbing ease. As everyone’s trunk size is personal, and not really a function of height (tall people can have long or short legs), then this is a piece of kit that you really do want to try on before buying.
Some rucksacks have an adjustable waist-shoulder length. These can be pretty expensive, though, so if you can find one that fits without having to be adjusted, then great.
The next measurement to look for is head-neck swing clearance. When the pack is on, you should be able to roll your head around a full loop, shrug your shoulders and otherwise have complete freedom of movement. If it touches your pack at any point, it’s took high or too close (strapped in.)
A another adjustment you want to look for are the webbing straps that hang from the body of the rucksack. These are the dangly bits you see when you’re walking behind someone. Depending on their position these can shorten the height of the rucksack, or make it thinner round the middle. This is a useful thing to be able to do. If you don’t happen to have exactly the right amount of kit to fill the rucksack, you’re likely to have empty space in there. Empty space lets your stuff move about, which can put your centre of gravity out and lead to backpain. So if you can tighten the whole thing up until it’s straight and packed, then great.
It’s also much easier to have a larger pack that you reduce in size with these straps than a smaller pack you try to stuff everything in to. Bear in mind that kit, when wet, usually expands in size. Even camping stoves and cameras seem to take up more space when everything’s sodden. So a pack that was nice and tight when you started can easily end up way too small when you’re out and about.
So when you get through trying out the rucksacks available and find the ones that fit you, have webbing adjustment, allow for head movement, and have a good, comfortable waistband, then you’re probably down to the last two in the shop. That’s the point where you can start to look at the other bits and pieces.
For example a top zipped section is useful to keep must-get-at-quickly stuff. Separate side pockets are useful for keeping nasty stuff away from nice stuff. For example, one side pocket stuffed with old smelly clothes keeps them away from your sleeping bag. They can also be used to store food away from camping stoves/parafin, and putting water bottles in.
Some rucksacks have neat little pockets for camel backs. These are flexible waterbags with a long drinking tube, which you put on your back and constantly sip from. They can taste disgusting, but are great for keeping you hidrated. So if you like these, having a rucksack that has a separate wee pocket for them is great. Even having a little hole to poke the drinking tube through can be useful.
Some rucksacks come with mesh back-pieces that let air whip round your back. This can be quite nice. Others also come with flexible back-plates that can be pulled out, moulded, then stuck back in. I’ve heard of instructors claiming you can use these as emergency splints, but the ones I’ve seen would be rubbish at that.
Then finally come with little loops, hard points, lanyards and webbing straps. These are meant for hanging stuff from, stuffing stuff down, tucking stuff in. They can be quite useful, but I’d use them sparingly. Swing too much from them and you may as well walk along yodelling or singing the wild rover. And never, never consider them an appropriate place to dry your pants on.