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, tried to remove the author, date and comments classes, and remove the entries to hide those items from the “meta bar”, from css, but it shown them again, so I’m guessing I did not do something right with content-extensions.php .