Hi Kerstyn,
I see. Currently the only way for you to set the character limit is by modifying the core code directly within easydiscuss. You can try to go to your /components/com_easydiscuss/classes/composer.php and at approximately line 132 add one more html attribute called "maxlength" as you can see below.
$html .= '<textarea class="dc_reply_content full-width" name="dc_reply_content" class="full-width">' . $this->content . '</textarea>';
// Replace above with below and modify the value of the maxlenght according to your need
$html .= '<textarea class="dc_reply_content full-width" name="dc_reply_content" class="full-width" maxlength="150">' . $this->content . '</textarea>';
Hope these help.