I added the exact code that you offered in the post Show current page featured image in sidebar. The featured image is displayed, but the size of the image is unaltered from the original.
I am creating a child theme for "responsive", and this image is for a page, not a post. I used this code as the first thing in functions.php:
add_image_size( 'sidebar_post_thumbnail', 100, 9999, true );
When I insert the image as the featured image, my custom 100px size does not show up as a radio button. Also, I clicked the radio button for the default thumbnail size (150x150). That also had no effect.