function limit_upload_size($file)
{
    // Set the desired file size limit
    $file_size_limit = 1024; // 1MB in KB
    // exclude admins
    if (!current_user_can('manage_options')) { //如果不是管理员
        $current_size = $file['size'];
        $current_size = $current_size / 1024; //get size in KB

        if ($current_size > $file_size_limit) {
            $file['error'] = sprintf(__('ERROR: File size limit is %d KB.'), $file_size_limit);
        }
    }
    return $file;
}
add_filter('wp_handle_upload_prefilter', 'limit_upload_size', 10, 1);

if (!current_user_can('manage_options')) { //如果不是管理员
    add_filter('upload_size_limit', 'wpse_228300_change_upload_size');
}
function wpse_228300_change_upload_size()
{
    return 1000 * 1024;
}

使用上述代码后：