Try on both boots, while standing, with the socks worn for hiking. Try on several models or sizes if necessary. Check two comfort points: the seat of the heel (the heel does not lift, or very little) and the front of the foot going downhill. The toes don’t touch the end. Avoid all pressure points when trying shoes on. Don't hesitate to walk around the store. Wear your boots in gradually during your first hikes to break them in.