Limit attachment caption characters

do you have a custom meta field associated with the image? if so place this in your functions.php:

<?php
$trim_length = 21;  //desired length of text to display
$custom_field = 'your custom field key here';
$value = get_post_meta($post->ID, $custom_field, true);
if ($value) {
  echo '<p>I want to use: ' . rtrim(substr($value,0,$trim_length)) . '...</p>';
}
?>

this will limit the character count in any meta field just place your meta key in $custom_field=”;

hope this helps cheers.