The front panel, like the back panel, has a 3.5" overlap on the side to cover the stud of the side panels. For the sake of neatness, you may wish to have a 5" strip for the opposite side (where the door hangs) to cover the join.
The location of the interior studs depends upon the width and position of the door. Build the box, first - the outermost studs - and then carefully measure the rough opening for the door, making it 1/2" wider than the door itself. Then allocate the inner studs as appropriate. The plywood/insulation sheath goes right up to the doorway, covering the inner door stud. The door will hang from the outer stud, and swing outwards..