in short: you are stuck with the need to emply an external tool.
But it should be easy to export the existing cover files (there is an action for that),
Resize them with an appropriate tool like gimp
reimport the files (there is an action for that).
In general: how often do you modify the size of cover art for a huge amount of files? I would say that this happens once in a lifetime of a file: then, when you tag it to include it in your collection.
1.Select the files with covers and export them to a defined folder with their filenames.
2.Import covers for mp3 files from folder according to their filenames.
You dug up a topic from 9 years ago - and although the description of the cover export and import is OK, the solution for the topic has changed.
In the meantime 2 actions have been added that deal with pictures: Adjust cover which changes the dimensions and the mime type
and Set cover properties which sets the cover type and possibly a description.
Both was not possible 9 years ago.