Hi Tony Parke,
You have to edit the core code in "index.php" file (mod\chat\gui_header_js\index.php).
There is you can find these lines:
<frameset cols="*,200" border="5" framespacing="no" frameborder="yes" marginwidth="2" marginheight="1">
<frameset rows="0,0,*,50" border="0" framespacing="no" frameborder="no" marginwidth="2" marginheight="1">
<frame src="../empty.php" name="empty" scrolling="no" marginwidth="0" marginheight="0">
<frame src="<?php echo $updatemode ?>.php?<?php echo $params ?>" name="jsupdate" scrolling="no" marginwidth="0" marginheight="0">
<frame src="chatmsg.php?<?php echo $params ?>" name="msg" scrolling="auto" marginwidth="2" marginheight="1">
<frame src="chatinput.php?<?php echo $params ?>" name="input" scrolling="no" marginwidth="2" marginheight="1">
</frameset>
<frame src="users.php?<?php echo $params ?>" name="users" scrolling="auto" marginwidth="5" marginheight="5">
</frameset>
By editing cols and rows attributes in frameset tag you can do it.
Regards
Brami