With the Minimal theme there are some options which can affect the thumbnail sizes. For example, changing the layout option to be full width or not and/or setting the thumbnails to be under the main image or to the right in the product page options could be all you need.
If that does not do the trick and you need them to be reduced further take a look at our Design Policy. Since Minimal is a Shopify made theme, it is supported by us.
If you still have any remaining design time then do not hesitate to reach out to our Support Gurus so we can forward this request to our Theme team to make the change! :)