Alejandro,
I'm assuming you're talking about a Page Tab and not the App Canvas. The App Canvas can do automatic width resizing as-is and Autotune should configure it that way. Unfortunately, there's no way around the 810px max-width for Pages. I'm not sure what you mean by 'autoresizing', but JFBConnect won't be able to automatically smush your site to a shorter width. For that, we'd recommend you copy your current template, make the width 810px, fix anything that breaks, and set that as the Page template in JFBConnect.
For height, you'd want to disable the Automatic Resizing option in JFBConnect. Then, you'd need to add the following Javascript to your site:
[code]<script type="text/javascript">FB.Canvas.setSize(800);</script>[code]
800 is the px of height you want. We can help you add the code above if you're unsure how/where to do that.
Beyond that, I don't think I'd need your admin credentials for anything. There's not much more we can do here. If you do want to send them though, you can Private Message them to me. You can PM me by clicking the arrow under my picture (<----) and hitting the mail icon.
Hope that helps, and good luck,
Alex