functions.php

<?php

/**
 * Teaser Thumbnail
 * Overrides the thumbnail option set in Genesis > Theme Settings
 * if on a teaser post
 *
 * @param string $opt
 * @return string
 */
function be_teaser_thumbnail( $opt ) {
	if( in_array( 'one-half', get_post_class() ) )
		$opt = 'be_teaser';
	return $opt;
}
add_filter( 'genesis_pre_get_option_image_size', 'be_teaser_thumbnail' );