When doing the slides for my optional parameter video, it never even occurred to me that PowerPoint may be able to accommodate aspect ratios other than 4:3.  While watching my video after it posted to YouTube, I started wondering if PowerPoint would support 16:9.  Sure enough, from the Design tab, click “Page Setup,” and the first configurable property, “Slides sized for,” lets you pick from several aspect ratios, including 16:9.

Wish I’d thought to look for that sooner.