You can make use of this code in the X > Theme Options > Global CSS

.woocommerce-product-gallery .flex-control-nav.flex-control-thumbs img {
    height: 80px;
}

Feel free to adjust the height and leave the width out of it because it is set as auto. Having to set the width would get the image squeezed or distorted.

We would loved to know if this has work for you. Thank you.