Adding a custom field or metabox to the post-thumbnail widget?

Just filter admin_post_thumbnail_html and append your checkbox HTML:

add_filter( 'admin_post_thumbnail_html', 'wpse_71501_thumbnail_options' );
function wpse_71501_thumbnail_options( $html )
{ 
    return $html . <<<html
<p>
    <label for="big_thumbnail">
        <input id="big_thumbnail" name="big_thumbnail" type="checkbox" />
        Use big thumbnail
    </label>
</p>
html;
}

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