Are you looking for something like this: http://mhsinfo.mckesson.com/PI_Webinar_Series_Registration_2013.html? Or do you really mean 100% of the page?
For us, our template includes a <div> that only goes a particular width. If you literally wanted it to span the full width of the page, I'd imagine you'd have to create a new template. This Webinar totally ROCKED and should be able to help you if you need to create a new template: https://community.marketo.com/MarketoArticle?id=kA050000000L5L3CAK.
I hope that helps!
Anyone know where that original webinar landed? With the new community, I keep getting redirected to the main product docs page.