You can figure out what you are going to take, get it all together and do the box thing. That is pack it all down nice and tight in a box, measure the box (inches by inches by inches), then multiply inches times inches times inches. This will give you cubic inches required.
I’d go will a little bit bigger size rather than smaller. You can always stuff the extra room (if any) full of popcorn from Walmart, weighs very little and very yummy on the trail and you’ll make lots of hiker friends with it.
Go with shoulder straps and waist belt and something a little durable. Heavier packs are more solid and carry the weight a lot better and you will hike more confortably with them.
Water bottle holders on packs are great. If not used, you can stuff them full of snickers bars, more friends on the trail for sure. Also those hydration systems freeze up and tend to spring leaks out there sometimes. I’d have a back up water bottle for sure.
See you out there. :cheers
Maintain