Is commissuroplasty the right procedure to get wider (horizontally) lips? (Photo)

There are surgical and non-surgical approaches to lip augmentation. Non-surgical approaches include botox (slight lift, reducing gum show) and filler (volume restoration). Surgically, you can have two separate incisions under the nose (italian), one incision under the nose (bull horn), one incision at the red/white upper lip junction (gull wing) or two separate incision at the corners, or a combination, depending on your desired look and anatomy. My preferred surgical technique is called the Elelyft, which is a version of the bullhorn approach. 
In your situation, there is no great surgery to widen the mouth without risk of significant scarring. Certain other aesthetic changes might be feasible especially non-surgical but an in-person consultation would be best.
Consult with an experienced lip surgeon for a complete range of options that are appropriate for your situation.
Gary Linkov, MD
Lip Specialist / RealSelf Lip Lift Medical Reviewer
Manhattan

As a general rule the ideal mouth width is at a vertical line dropped down from the pupils. How much distance that line is from your mouth corners can not be known by just looking at a picture. But in most widening surgery I don't like to move the position of the mouth corner further than 7mms per side from its natural position to avoid undue scarring and retraction.. 

Commissuroplasty is a procedure that is only performed in people for severely injured mouth such as after burns or electrical injury. It is difficult to tell what's going on with your face and I would recommend that you have an in-person consultation with a plastic surgeon. Perhaps there is something that can be done to create better proportions, but you need to be seen. I hope this helps and good luck from New York City plastic surgery