Change thumbnail / child image size on product page

-- HikaShop version -- : 4.0.1
-- Joomla version -- : 3.9.2

Where/how do I change the default size of the thumbnail/child images on a product page?

When I enter a size in the 'thumbnail' field under 'System / Main / Images' nothing happens. If I change the size for 'product page image' in the same pane I can change the size of the product image on the product page.