Hello,

To increase the size of the add to cart button on the product page you need to add the following Custom CSS in the Global Custom CSS area under Theme Settings >> Custom CSS.

.product-image-summary .single_add_to_cart_button {
font-size: 20px;
line-height: 18px;
padding: 12px 100px;  
height: 70px;
}

You can change the padding, font-size and height according to your requirements.

Best Regards.