Using Advanced Custom Fields Pro and the documentation mentions the ability to change Gallery thumbnail preview size…
“The WP image size which is displayed when editing the images. Defaults to Thumbnail.”
I don’t seem to have that setting/field when creating a new field group. Has it been removed or is something else going on?
It appears that the documentation is a little behind reality. That setting was removed some time ago. I believe it was removed when version 5 was released, but not sure. I don’t remember it every being a setting so it was a really long time ago.