Hello,

Please add this code to the Theme Settings > Custom CSS:

body.product-image-summary .single_add_to_cart_button {
    width: 300px;
}

@media (max-width: 768px){
body.product-image-summary .single_add_to_cart_button {
    width: 100%;
}
}

Replace 300px for the width you like.

Best Regards