Find the maximum height by measuring from the shoulder line to the waist line and add two inches. The shoulder line is where the backpack straps will actually rest on the body, about half way between the neck and shoulder joint. The waist line is at the belly button. The backpack should fit two inches below the shoulders and up to four inches below the waist so adding two inches to our measurement will give us that.
The width of the back can be measured at a number of locations. Each with different results. For a backpack we want the core and hip muscles to carry the weight so we want to keep it centered between the shoulder blades. Measure between the ridges of the shoulder blades. An extra inch or two here is acceptable.