Hello,
1) There is no default image size for the image that is used as the page title background. You need to use a high quality image according to your needs. If you want to ask anything else then provide with some relevant screenshot of the specific area to check it myself and help you out accordingly.
2) Please deactivate and delete the current WPBakery page builder, then navigate to Appearance > Install plugins > install and activate the recent version.
If you have any problems, please send us your site admin panel access in the private area.
Best Regards.