Smaller version of header for one page

my observation is that it would be very hard to determine for one page a shrinking if you do not have shrinking header on enfold options page.
Guess it has something to do with embedding the scripts needed and to get rid of html class – set from beginning. Maybe Guenter knows a way to get this way around.
But as i see – there is a shrinking header – that is possible to get rid of for one page

i would use the filter a bit different. – Replace that code above ( save a copy of that snippet ) :

function avf_header_setting_filter_mod($header) {
  if ( is_page(21) ) {
    $header['header_layout']      = 'logo_left main_nav_header menu_right';
    $header['header_sticky']      = 'header_sticky';
    $header['header_shrinking']   = 'disabled';
    $header['header_stretch']     = 'disabled';
    $header['header_size']        = 'custom'; // slim, large or custom
    $header['header_custom_size'] = '45' ; // if custom size : integer value in px
    // on default we do add here classes by .= but in this case
    $header['header_class'] = " av_header_top av_logo_left av_main_nav_header av_menu_right av_custom av_header_sticky av_header_shrinking_disabled av_header_stretch_disabled ";
  }
  return $header;
}
add_filter('avf_header_setting_filter', 'avf_header_setting_filter_mod', 10, 1);

PS : on dropdown custom pixel value option less than 45px are not possible – on that snippet above you can even choose less than 45px

This way with page array

if( is_page ( array ( 21, 8675, 13792, 14614 ))){