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I took a quick look at the API and unless I missed something it doesn't seem possible. Calibre doesn't pass the size of the existing cover when invoking metadata plug-ins.
To make that possible Calibre would need to extend its API. But as I explained, if a change is needed in Calibre to enable this feature than it's actually better to implement the feature itself in Calibre: the user experience can potentially be much better than doing it on the plug-in side, which doesn't have control of the UI and how results are presented.
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