can't do it. I have never found a way to do this. That is one of the main reasons why it is not a good idea to try and put the board into an iframe.
If you want to wrap the board in your main site, just edit the overall_header.tpl file for your style with the html from your main page header and menus etc.
Well if you take a good look at this page you'll see this has nothing to do with board, that's my server's stats page. Simple question is: did anybody found the way to place iframe from another domain with out scroll bars and have height auto adjusted
and the simple answer is ... no.
that is what I said above. it doesn't matter what you are putting into the iframe, it won't auto adjust as far as I have been able to figure out.
also, if this is not related to your board, then why are you asking it in the phpbb MOD request forum?