IMHO worrying about load for such a small number of downloads (less than 25 millions) is silly. Just use a sensible, common size, for instance 800px wide which is the default. There's also an outdated https://www.mediawiki.org/wiki/Requests_for_comment/Standardized_thumbnails_sizes/thumb_sizes_requested Alternatively you can download/mirror the whole https://archive.org/details/wikimediacommons and do some scaling yourself. :P
Nemo