Can you make someone's mouth wider and bigger?

You can use hyaluronic-acid based dermal fillers such as Volbella, Juvederm and Restylane to make your lips look larger and even create the illusion of more width. However, the actual opening of your mouth remains the same. If you’re unhappy with your looks, consulting with a board-certified dermatologist can help you learn more about possible treatments that could make you feel better and more confident.

Lip augmentations with fillers is a great option to improve the fullness, suppleness and symmetry of the lips which would make your mouth appear bigger. Results can be very natural-looking and will feel very natural. I prefer to use a micro cannula to inject the lips to prevent bruising. Look for a cosmetic dermatologist or plastic surgeon who is an expert in fillers to discuss your desired results and concerns. 
I hope this helps.

dh2002721.  I’m not sure exactly what you are asking. Photos would help, however lip augmentation with fillers will increase the size of your lips and make your mouth appear larger. However there is no way to physically or surgically alter your mouth opening to make it larger. Seek consultation by a board-certified plastic surgeon in your area to pursue options for you. Good luck.