Can’t change image size from post “Show current page featured image in sidebar”

The size options have no effect on the featured image, those only apply when inserting an image into post content, and furthermore, you won’t see custom sizes show up there, just the default, built-in ones. The size that is output in the example you linked is controlled by the call to the_post_thumbnail( 'sidebar_post_thumbnail' );

Another thing to note is that if you’ve added a new image size after you’ve already uploaded images, those new sizes won’t automatically be created. you either have to re-upload the images, or use a plugin like regenerate thumbnails.