The size of the small dog door opening is about 6 inches in width and 10 inches in length. In order to find out that the small dog dooris suitable for your animal, you should take a large box and a make a hole in it of about 6* 10 inches. This hole should be 2 inches above the base of the box. Now place your animal inside the box and if he is able to come out of that hole easily then, this small dog door is best for him.Two different small dog doors are offered by this company:Small Dog Door for Door : This door can be fixed into the front door of your house. The door thickness should be about 1 3/4″. This dog door has an additional feature of security panel as well. The price of this small dog door is $199. The overall size of the door is 13inches by 17 inches.Small Locking Pet Door for Wall: This door can be fixed into a wall. It is installed just like window. You just have to insert the blocking and then the frame of the door. The flaps which are normally fixed into the walls do not have a security panel but, this door has a security panel which can be locked also. The price of this door is about $159. The overall size of this small dog door is 11inches by 15 inchesBoth of these doors are shipped free by the company. Warranty, guidelines for installing the dog door and training your pet are provided with the product.