Can I Bulk Edit/Change the Size Guide in All Campaigns?
Many of the campaigns default to a size guide at the following location: https://yourdomain.com/wp-content/uploads/2016/09/blank-size.png. You could replace that file with your custom guide. You could rename the current file in your file manager in cpanel, then name your custom guide blank-size.png and copy it to that folder. It should replace the current one with yours in all the campaigns that have that file as the guide. The size of this size guide is 835X2025.
Attached is the standard size guide.