When you feel like wanting to show the size of photos below the thumbs, Coppermine sometimes isn’t of help. Despite having a few functions not everything is tailored to your need.
So here’s a quick solution for showing it.

Locate the functions.inc.php file inside the include folder of your coppermine installation.

Find this line
$rowset[$key]['caption_text'] = $caption;

Above it you need to add another line
$caption .= '<span class="thumb_caption">'.$row['pwidth'] . ' x ' . $row['pheight'] .'</span>';

And now the size of your files will show.

Please Note: this coding tutorial is based on one that was available on coppermine forum, which I can no longer find on there