As with anything, it's a good idea to shop around when buying a backpack. One size definitely doesn't fit all! Different kinds of backpacks suit different body types and different kinds of travel / adventure. For example, they're available in different sizes which suit different size people and the kind of journey you'll be doing. For example, if you were doing a two-week hike in the wilderness you'd need a different kind of pack than if you were spending a year travelling around Europe.
I'd recommend going to a hiking shop such as Kathmandu, Mountain Design or Paddy Pallin, and speaking to some of their staff. Most people who work at these stores are experienced bushwalkers and travellers and would be able to give you expert advice. The packs may be a bit more expensive than elsewhere but they'd be of top quality and perfect for your specific height and strength, and the kind of travel you're planning.