If I understand your question correctly, you would set a your preferred featured image/thumbnail dimensions either in Settings or, if you have or need a new special image type, by adding it to your functions PHP – something like:
add_image_size( 'product_preview', 55, 55, true ) ;
In order to retroactively crop the old images, pretty much standard practice these days (1 million users) seems to be to use the “Regenerate Thumbnails” plug-in, which also regenerates the other image sizes. https://wordpress.org/plugins/regenerate-thumbnails/