For now you cannot do that in Grome. We plan to add it in a future version of the editor.
In the mean time you can create an utility plugin that takes a zone data and creates a new one with the different size, while deleting the source zone. The data extraction code should be similar with the code used by the exporters, while the new zone creation is identical with creation of zones by the import plugins. We will be more than happy to help you with the implementation in case you have issues.
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