AutoAdSense just plasters large ads all over the page no matter what you do. There used to be ways that worked to limit sizes but they no longer work. Nothing works now. Google knows best.
Do yourself a favor: Turn off AutoAds and set up individual ads and sizes that you can apply with whatever ads manger you use, including the Xenforo built in one. You can use conditionals with the Xenforo version or options with the others.