Apple TV sends out a 16:9 image, always. Keep that in mind, and configure the other devices to match, as well as your documents!
The projection size is fully controlled on the projector, not on Apple TV, nor Mac. You can set the Mac screen mirroring to match the Apple TV screen for aspect ratio, rather than the aspect ratio of the built-in screen (System Settings﹥Displays﹥Optimize for﹥Apple TV); that may help in avoiding projecting black bars or pillars. [Not needed if set to separate display (=2nd screen), which Keynote or PowerPoint will happily use for the full screen presentation.]