// change thumbnail size
function jetpackchange_image_size ( $thumbnail_size ) {
 $thumbnail_size['width'] = 250;
 $thumbnail_size['height'] = 250;
// $thumbnail_size['crop'] = true;
 return $thumbnail_size;
}
add_filter( 'jetpack_relatedposts_filter_thumbnail_size', 'jetpackchange_image_size' );