800x600 is fine. I think the max red is 1280x720, so best to stay under that.

I started making my game in 1024x768 aiming for the old arcade 4:3 screen ratio.

But now I'm assuming the screen ratio will be 16:9 based on the max res being 1280x720?

The players will see it on a widescreen monitor, and the audience will see it on a projector. I believe the projector is 1280x720 so it will probably mirror that on the monitor, even though the monitor is capable of a higher resolution.

For my submission I am hiding the Unity setup screen from my .exe so it auto-boot but you can CTRL+Click a Unity .exe to configure resolution and that'll be saved on that computer.