My shoe is roughly 12" toe to heel, so I am able to step off the needed dimensions. I have seen other umpires use the players bat to approximate 36".
Second, Rule 6.03 states:
The batter’s legal position shall be with both feet within the batter’s box. APPROVED RULING: The lines defining the box are within the batter’s box.
Many batters are instructed to move closer to the plate and some will take a position where a foot or both feet are over the line and within 6” of the plate. This is an illegal position and the batter should be instructed to take a legal position in the batter’s box.
Third, Rule 6.06 states:
A batter is out for illegal action when—
(a) He hits a ball with one or both feet on the ground entirely outside the batter’s box.
Comment: If a batter hits a ball fair or foul while out of the batter’s box, he shall be called out. Umpires should pay particular attention to the position of the batter’s feet if he attempts to hit the ball while he is being intentionally passed. A batter cannot jump or step out of the batter’s box and hit the ball.
Note that the batter is out by hitting the ball regardless whether the hit results in a fair or foul ball. Note also that the MLB rule makes no reference to the batter’s foot touching the plate.