How to enforce minimum height and width for image while resizing anything larger?
Reformatting an image won't happen on upload, it will happen on display of the content. If you want to reject anything less than 480x480, set that as the minimum in the field's setting and don't set a maximum size.



To resize the image, be sure there is an image style set to resize an image to 480x480 (which is included by default in Drupal 8). Then under the "Manage Display" field of the content type, click the gear next to the image field and select the desired image style that you want displayed.