Any decent hiking pack is adjustable in several different ways. But not every pack can be adjusted enough for every hiker, so please do your research before you buy.
Hiking packs can be adjusted to accommodate various torso lengths, often adjustable as many as six inches. This adjustment usually involves adding or reducing the distance between the shoulder straps and the waist belt. Ideally, you can find a hiking pack that fits you perfectly in its middle setting, e.g. a pack that accommodates torsos between 18" to 24" while your ideal setting is 21". That's the case because minor adjustments during the course of a hike can shift where you concentrate the load, from waist to shoulders, giving various parts of your body little breaks from time to time.
The waist belt itself is critical, as most of a pack's weight is carried on your hips. Make sure you choose one that fits you comfortably while still offering plenty of opportunity for tightening or loosening.
Also, consider the type of padding on the back. Softer pads may seem like a draw at first, but they might also soak up a sweat and hold heat, for example.