Full image on single post

try both inside your child-theme functions.php:

function avf_template_builder_content_postimage_mod($content = ""){
  if(  is_singular('post') || is_singular('portfolio') && ( '1' != get_post_meta( get_the_ID(), '_avia_hide_featured_image', true ) ) )  {
        $featuredImage = get_the_post_thumbnail( $the_id, 'original' );
        $content = '<header class="entry-content-header"><div class="page-thumb">' .$featuredImage. '</div></header>' . $content ;
      }
  return $content;
}
add_filter('avf_template_builder_content', 'avf_template_builder_content_postimage_mod', 10, 1);

//image-sizes:  widget, square, featured, featured_large, extra_large, portfolio, portfolio_small, gallery, magazine, masonry, entry_with_sidebar, entry_without_sidebar, medium, large

function custom_post_featured_image_link( $image_link, array $current_post, $size ){
  if(  is_singular('post') || is_singular('portfolio') && ( '1' != get_post_meta( get_the_ID(), '_avia_hide_featured_image', true ) ) )  {
    $image_link = get_the_post_thumbnail( $current_post['the_id'], 'original' ); /**** or medium, square etc. ***/
  }
  return $image_link;  // echo $image_link;  if you want to get rid of link function
}
add_filter( 'avf_post_featured_image_link', 'custom_post_featured_image_link', 10, 3 );

these snippets have that check if on the editor mode – on the right side in the layout box “show/hide featured image on single post”
so they will respect that setting.

PS that comment : echo $image_link; if you want to get rid of link function. –
Most people don’t like the fact that the featured image in single posts has a link to their own post. I think so too – that’s superfluous

But – removing that link function will move the feature image out of the article container. That means the image is above all – title is beneath.
So it might be better to have that link – and preserve enfold structure – but hamper only the pointer-event on that image

or you take – mikes snippet to redeclare these formats – but with setting of crop to false!
( but the solution above is more elegant, and also shows the full resolution )

function avf_customization_modify_thumb_size( $size ) {
      $size['entry_without_sidebar'] = array( 'width' => 845, 'height' => 9999 , 'crop' => false );
      $size['entry_with_sidebar'] = array( 'width' => 1210, 'height' => 9999 , 'crop' => false );
      return $size;
}
add_filter( 'avf_modify_thumb_size', 'avf_customization_modify_thumb_size', 10, 1 );

the limitation value is not the width value – so you got smaller images. This is as Mike mentioned for the new images only – but if you recalculate the thumbs even the older posts will have that new size (without cropping)