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 There's also an outdated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