The length of your back should dictate the height of your rucksack. If you feel as though you are being tipped forward when it is full, it is too high.
Even if you have some type of seat fastened to the top, it should feel comfortable to carry. And it should not impair climbing, jumping over streams and ditches, getting through thicket or maybe even crawling on hands and knees.
If you do not have to carry so much, a soft rucksack will suffice. If you are on a long hike or carrying a substantial load, a framed rucksack is recommended. It should also have a belt, to relieve your back and transfer the load to your legs.