functions.php

<?php //Don't include php start tag

//Remove sidebar from non-singular pages
add_filter( 'genesis_pre_get_option_site_layout', 'custom_fullwidth' );
function custom_fullwidth( $opt ) {
  is ( !(is_singular() || is_admin()) ) { //is_admin to prevent setting Full Width on Genesis> Theme Setting everytime you visit.
    $opt = 'full-width-content';
    return $opt;
  }
}