Preview size is only available for the image field. The main reason that this is not available for the gallery has to do with the way the gallery is shown in the admin, the fact that it’s responsive and the drag/drop sorting feature. There’s quite a bit of CSS and JavaScript that depend on the images being a predictable size and square. I’m guessing that it would be a large amount of work to allow for multiple sizes.
The image that ACF uses is actually whatever you site has set for the “medium” size. The container for each image in the gallery is set by ACF with parts of the image being hidden.
