Hello,
You'll have to open up the php file for the s5 search module, look for:
<input type="text" onfocus="if(this.value=='<?php echo $search3text; ?>') this.value='';" onblur="if(this.value=='') this.value='<?php echo $search3text; ?>';" value="<?php echo $search3text; ?>" size="50" class="inputbox" alt="search" maxlength="20" id="mod_search_searchword1" name="searchword"/>
NOTE: This is only for the built in google and yahoo searches if you are trying to change the Joomla search length this is done via the module admin side.
Regards,