Allow author to choose his featured image size

You could add a meta box to the post add/edit screen below the featured image box to allow selection of size via a dropdown select menu. You could generate the list from available sizes (via get_intermediate_image_sizes), or just use your small/large strings. The data would be saved as post meta and you would simply get_post_meta() to get the selected size, then pass that result to the thumbnail function as the size parameter.