Stretchers come in all shapes and sizes. From men's small, medium, and large, to the same for women but with an added, high heel pitch, and pointed to rounded toe shapes. One thing that they all have in common is they only stretch width.
Most people just need a General Stretch to help them break the shoe in. The size is determined by the innersole of the shoe which does not change. Only the upper is being bubbled out to take the pressure off of your foot.
If you have a bunion or a specific spot on your shoe that hurts, it can be pushed out harder just in that area to give you relief. This was done to the boots in the photograph. As the shoe is worn, this bubbled out area will disappear and the boot or shoe will no longer be painful to wear.
If the shoes are leather or suede and you want to break them in yourself, you can spray them with Kiwi Shoe Stretch Spray and walk around in them wet. It speeds up the break-in process. This spray is also used during the stretching process with wooden stretchers.