Woocommerce Product Gallery Thumbnail size

Since one of the latest updates the product image gallery thumbnails are distorted, because woocommerce removed the option to set the size manually and decided to use 100x100px for some reason – probably this is an “improvement”. Well for me at least, this made the product gallery look ugly (i suppose i am not the only one) using the Flatsome theme. Now my question, how to adjust the size of the product gallery thumbnail images from 100×100 to a more suitable size ?

Best Regards

Dan