Use the 'Upload a larger size' link on the photo (or edit) page and submit the size you want (or select the preferred option on the page). This will overwrite the original selection although there may be a brief period when the originally selected size will be available due to caching and the wait before approval.
Be aware that if the very large version has been on the site for a while it may already have been downloaded from the site as a CC licensed image.
Note: if you did not want to license a larger size at all you will need to go through the same process but using an image marginally larger than 640 pixels wide (641 is enough) in order to remove the larger size options.