how to change width and height of shoutbox ?

If you insert the script as provided, you will get 100% height, 100% width shoutbox and certianly this is not what you expect.

If you want a 320px, 240px shoutbox, you just have to insert the shoutbox script into an paretnt element whose size would be 320px x 240px

ex:
<div style="width:320px; height:240px">
Your script here
</div>