多亏了Rarst给出的答案，我才能够找到this 在Wordpress文档中。。。

function get_image_sizes( $name ) {
    global $_wp_additional_image_sizes;
    $sizes = array();
    $get_intermediate_image_sizes = get_intermediate_image_sizes();
    // Create the full array with sizes and crop info
    foreach( $get_intermediate_image_sizes as $_size ) {
            if ( in_array( $_size, array( \'thumbnail\', \'medium\', \'large\' ) ) ) {
                    $sizes[ $_size ][\'width\'] = get_option( $_size . \'_size_w\' );
                    $sizes[ $_size ][\'height\'] = get_option( $_size . \'_size_h\' );
                    $sizes[ $_size ][\'crop\'] = (bool) get_option( $_size . \'_crop\' );
            } elseif ( isset( $_wp_additional_image_sizes[ $_size ] ) ) {
                    $sizes[ $_size ] = array( 
                            \'width\' => $_wp_additional_image_sizes[ $_size ][\'width\'],
                            \'height\' => $_wp_additional_image_sizes[ $_size ][\'height\'],
                            \'crop\' =>  $_wp_additional_image_sizes[ $_size ][\'crop\']
                    );
            }
    }
    // Get only 1 size if found
    if ( $size ) {
            if( isset( $sizes[ $size ] ) ) {
                    return $sizes[ $size ];
            } else {
                    return false;
            }
    }
    return $sizes;
}

<?php 
    var_dump(get_image_sizes(\'custom-size\'));
?>

array(3) {
    ["width"] => int(250)
    ["height"] => int(250)
    ["crop"] =>  bool(true)
}