Custom Size Thumbnails in Version 2.0

Great plugin!
In prior versions I was able to set a custom size for the displayed thumbnail, an option unavailable in the update. Changing the settings for the thumbnail size in Settings > Media had no effect on the size of the displayed thumbnails.
There is a link in the changelog for “how to add image sizes”, but it links to an empty page. Can you please update the link or explain here how to add a custom image size in the new version? For the moment, I have reverted back to version 1.8.1
Thank you so much!

https://ww.wp.xz.cn/plugins/ultimate-posts-widget/