You can add some line breaks in the template.
Admin CP >> Templates >> <yourtemplate> >> Private Message Templates >> Send
In the part where you see
You could add some linebreaks before {$message}
(line breaks are <br/> in HTML)
Admin CP >> Templates >> <yourtemplate> >> Private Message Templates >> Send
In the part where you see
<textarea name="message" id="message" rows="20" cols="70" tabindex="4">{$message}</textarea>
You could add some linebreaks before {$message}
(line breaks are <br/> in HTML)