How to make jaw muscle larger?

It is difficult: As with any muscle, they tend to become more plump when we use them and tend to atrophy (diminish) when we don't use them. We only use jaw muscles for certain things (chewing), so it's hard to 'not use them'. If you grind (bruxism) or clench your teeth at night, the muscles tend to become more prominent due to excessive use. See a dentist about a diagnosis and treatment.

Cosmetic?: I assume for cosmetic reasons, you want a "square jaw" look. If so, no good way through exercises (bruxism can make jaw muscles larger but at the expense of causing jaw problems). The best bet would be to see a facial plastic surgeon or oral & maxillofacial surgeon to do an augmentation or grafting procedure.