According to this post:
I can change the Resolution of the Slides to 16:9 by clicking Format - Page. However, I’m using Windows version 5.1.1.3, I 'm not able to find this option.
Not sure why it’s not a menu item (at least I can’t find it) but it is on the Standard toolbar. It’s a page icon with a wrench on it.
In v5.1, Slide > Page/Slide Properties… > Page tab > Paper format section > Format pull-down list. In prior series the menu was Format > Page…