see
Features you should definitely consider...
or
Batch Album Art Resizing
or
Feature Suggestion
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.