Between the front of the big toe and the end of the shoe, there should be about ⅜ - ½ of an inch space (roughly around the size of a normal thumb). This space should allow your toes to be able to wiggle freely inside the shoe. The heel should feel snug and not be able to slip out of the shoe while walking. The area around the top of the foot should feel secure with a little bit of tightness (but not so tight that it causes pain while walking/running).