Got to try it out today, but it doesn’t work.
I made a child theme of Travelify, and I followed what you said here and copied the whole function (mind you, I’m not at all knowledgeable in php, so I might have done it wrong)
Here’s what content-extensions.php has in the child theme :
<div class="entry-meta-bar clearfix">
<div class="entry-meta">
<span class="author"><a href="<?php echo get_author_posts_url( get_the_author_meta( 'ID' ) ); ?>"><?php the_author(); ?></a></span>
<span class="date"><a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( get_the_time() ); ?>"><?php the_time( get_option( 'date_format' ) ); ?></a></span>
<?php if( has_category() ) { ?>
<span class="category"><?php the_category(', '); ?></span>
<?php } ?>
<?php if ( comments_open() ) { ?>
<span class="comments"><?php comments_popup_link( __( 'No Comments', 'travelify' ), __( '1 Comment', 'travelify' ), __( '% Comments', 'travelify' ), '', __( 'Comments Off', 'travelify' ) ); ?></span>
<?php } ?>
<span class="size"><a href="<?php get_post_meta( get_the_ID(), $post_id, $single);?>"></a></span>
</div><!-- .entry-meta -->
<?php
echo '<a class="readmore" href="' . get_permalink() . '" title="'.the_title( '', '', false ).'">'.__( 'Read more', 'travelify' ).'</a>';
?>
</div>
Also that article you’ve linked isn’t clear in the case when you have multiple custom fields. How can you differentiate between them? For example, I want the painting size to be listed, and whether or not the paiting is in b&w or colour. How can I differentiate the two different custom fields between them? What I’m guessing the solution is what’s written on the “Implementation Details” section, but I can’t test that right now since it doesn’t work, so I’m asking you just in case 
Also, I suppose I could remove the author, date and comments classes, and remove the entries to hide those from css.