good fit is the most important thing! other than that, everything else is really up to personal taste. pack volume depends on how large you are and what kind of weather you'll be prepared for, but a 60L pack is a good all-around volume for most people and a wide range of gear. the difference between a 1-2 night trip and a 3-week trip is honestly not that much as far as pack volume goes; your base volume of gear isn't going to change, you'll just have more consumables to pack in on longer trips.
how do you like to organize your pack? different shape packs are good for different kinds of gear organization; do you want one big main pocket that everything gets stuffed into, or would you like a couple different pockets and divisions to sort between things you'll need during the day and things you'll need in camp at night? do you like to attach stuff to the outside of your pack? of course, those questions are hard to answer until you get a few backpacking trips under your belt and really sort out how you like to do things, so go with your gut when you're trying on packs and just accept that the first few trips will be clunky and awkward!
i lucked out in that the first pack i bought myself was a great fit for the organization system i've worked out over the years, and after living out of that pack for months and months worth of trips, i know where everything is and exactly how all my gear fits into it. but i also started out knowing that i'm the sort of person who likes lots of little pockets, and designating each pocket for a particular purpose, so having a pack that fit my body and also my ideas about storage was really important to me.