batters box
 

I know batters feet must be in the box to start. When would he be considered out of the box when hitting the ball? ie stride foot partially or fully out?

The foot must be on the ground completely outside the box (that is, outside the lines, which are part of the box).

In some codes (not OBR), the batter is also out of the box if he's touching the plate, even if some part of the foot is still on the line.