A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
In PowerPoint 2016 for Mac, it's already the default. 2011 was created before Microsoft made the switch to widescreen (13.333" x 7.5") format as the normal aspect ratio. So upgrading would solve this issue.
You can create a presentation with your preferred slide size and set it as the default. Here are Steve Rindsberg's step to do that: Create your own default presentation
One small wrinkle is that in PowerPoint 2011, only hundredths of an inch are respected by the program interface. So you can't create a slide that's 13.333" wide, though you can get very close. To really do it right, you should access a Windows copy of PowerPoint 2013 or 2016, save a blank presentation and open it in PowerPoint 2011. Save it, then set it as the default. The result will be a slide that exactly matches what newer versions of PowerPoint will create by default.