300x250 Ad floating right in message body


I was trying to place a 300x250 banner within the first post floating on the right and right-away I had a problem:


The 300x250 banner in the above screenshot doesn't retracts the quote dialog in the post! It however pushes the post content on the next line.. And if the posts height is less than 250px, which in this case it is; the banner doesn't alters the post height. I tried using 'min-height' but that doesn't works! (cause we need min-height for first post).

My code in 'ad_message_body' is:

<xen:hook name="ad_message_body" />
<xen:if is="{$post.position} % {$xenOptions.messagesPerPage} == 0 AND !{$message.conversation_id}">
<div style="float:right">
<--300x250 ad code was here-->

Please advice?