It is very important that you have sufficient overlap between the door and the sides of the doorway. If there isn't sufficient overlap, you will be able to see through the doorway when the door is closed, when looking at the doorway at an angle. This is because, with a sliding barn door, the door sits in front of the doorway, not in the doorway.
In order to have sufficient coverage of your doorway, it is recommended that you have 2" of overlap between the sides of your doorway and the sides of your door. In order find the correct width for your door, take the width of the doorway and add 4" (2" overlap on the right side, 2" overlap on the left side).
Ideal Door Width (Single Door)
Width of Doorway + 4" = Recommended Door Width
It is not as important to have sufficient overlap between the top of your door and the top of your doorway, as it is pretty unlikely that someone will be looking at the doorway from above (and be able to see into the next room). It is for this reason that we only recommend 1" of overlap between the top of the door and the top of the doorway. Because the hardware kit will hold the door 1/2" off of the ground, you can even get away with having the door the same height as the doorway and having 1/2" of overlap as opposed to the full 1".