I had set up my blog page with a sidebar initially and had large size set at 590 in Media settings. Got rid of the sidebar, changed media setting to 900 width and ran the AJAX thumbnail generator. It reported success but the posts didn't change size. I tried to change individual media without success.
I'm using Pexeto's Dandelion theme. If I write a new post, the image is the correct size.
Greatly appreciate any advice as I do not want to have to re-upload all my images to each post.