the metabox defaults to the value that was set through set_post_thumbnail_size, it'd be great if you could pass in the constructor either size or name and use that size where the new interface exists, i.e.:
set_post_thumbnail_size( 298, 167, true ); //the default size used in the thumbnail interface
add_image_size( 'athlete_thumb', 138, 174, true ); //the size i want in the new interface
new MultiPostThumbnails( array(
'label' => 'Athlete Thumbnail Image',
'id' => 'id_athlete_thumb',
'post_type' => 'athlete',
'size' => 'athlete_thumb' //new addition
));
besides its great! thanks for the effort!