I had the same issue and getting rid of the sidebar worked well. Afterwards I changed the featured image size to 1200x454 in the functions.php, then ran Regenerate Thumbnails.
The latter did not change the featured image though, in fact my image wasn't even shown at 792 (the original specs) width, but instead at 628 px width (which might have to do with the original image size). When I upload a completely new image, it shows up fine. Is there anything on existing images I can do, or is it easier to dump those and just upload them from scratch now?
The other issue I got after removing the sidebar on single posts is that any text I had between images now jumped up and put itself next to the first regular image of the post (which is 792 px wide), as if it's using the "empty sidebar" now. So instead of seeing some text, then 5 images stacked, then more copy, then some images etc, I now have the copy that was supposed to be between the first and second batch of images on the right side of the first image (where the sidebar normally is). How do I get rid of that and get the text back down?