Making new image size appear in Attachment Display Settings

I wanted to create a new image size for a re-design of our website. I followed the instructions given by Mike in this thread: https://kriesi.at/support/topic/image-size-proportions-for-the-blog-posts/?login_error and when I regenerated the thumbnails, the new size appeared. But I cannot get it to appear in the drop down menu in Attachment Display Settings.

I wondered whether that might be because it was missing the (‘avia_framework’) after the title name, so added that – but it still didn’t show up. Do you have any ideas what I might be able to do?