iCloud Keynote - Browser Slide Size Format

Is there a way to change the slide size within a pre-existing keynote presentation on the browser version of Keynote? There is an option to select a slide size when creating a new keynote presentation (Standard 4:3 vs. Wide 16:9) - is there a way to change between these two sizes with a pre-existing presentation? I want to change from Standard to Wide because the formatting is distorted.