Display Thumbnail Size in Admin Meta Box

In the Admin meta boxes for a post on my website with this plugin installed, the image shown in that admin meta box is the thumbnail size of ‘post-thumbnails’ for the featured image, but for the thumbnail named ‘Secondary Image’ it simply shows the full image that has not been cropped to a thumbnail size.

I have a custom thumbnail size of ‘vertical-sidebar’ that is a 16:9 vertical resolution that I use in my theme. I would also like to display this thumbnail size in the admin meta box for the ‘Secondary Image’ I added with this plugin. How can I do this?

https://www.ads-software.com/extend/plugins/multiple-post-thumbnails/