How big of a pack you need depends entirely on what you want to carry. So it makes more sense to focus on gear first -- then pick a pack that will carry it all comfortably. There are countless options, of course, but here's mine (pick apart and use what makes sense for you):
I wear one set of clothes (shirt or tee shirt, pants, undies and socks) -- and pack a second set in my pack. I add a UL wind jacket (Patagonia Houdini) and an insulation jacket (Montbell UL Down). I can be relatively comfy in all temps 30F and up. I pick quick-drying, no ironing synthetics or synthetic/cotton blend, simply styled, and in neutral colors. Mix and match mean four different outfits good for hiking and for dining at nice restaurants -- plenty enough. I wear one pair of comfy shoes good for both sidewalks and well-maintained trails and need no maintenance other than a quick wipe with a wet towel every now and then (Ecco Cross). If expecting colder temps, I pack a heavier down jacket (still very light and very compressible), plus gloves and a warmer cap.
Every other night or so, I spend 5-10 minutes hand washing my clothes and they dry by next morning. I never lug dirty laundry around the world -- so all I need is a small and light book bag.
Finally, I use a folder to somewhat compress my clothes for easy packing and to prevent them from sagging to the bottom of my pack.
Toiletry kit. (reasonable amounts only - replenish as needed in longer trips)
Misc kit - 3-4 days' supply of OTC medication (more if prescription), small LED flashlight, small notepad, a pen, etc.).
Flip flops for hotel and beach wear (Old Navy)
Small cable and lock -- seldom used, except securing my pack onto the luggage rack on long train rides so I can go to the bathroom or dining car, etc. without worry.
Guidebooks and docs, money, cards, etc.
Usually, total pack weight (including the pack itself) comes to 9-10 pounds. I can carry that all day without feeling it. This means when I arrive at a new town, I don't have to search out a hostel just to get the pack off. Oftentimes, I will explore a town and decide in the afternoon whether to stay the night or move on. I like the flexibility and spontaneity that traveling light provides -- more so than the dubious flexibility of "having everything available" when stores are all around.
Hope this helps.