Unfortunately once a document is created the size can not be altered - if possible, we would suggest that the required dimensions are known before creating a new document.
If your document has been created in the wrong size, you could create a new document in the correct size and copy the content over - if you open each design in separate tabs of the same browser you should be able to copy. If you have created your document using a blank design or fully editable design you can select a panel, then right-click and select 'copy selected panels'. Then right-click on the page in your other tab and select 'paste copied panels'.
Other articles in the ‘Creating a New Document’ series
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