Convert Units to Pixels and back

I need to create a plane in my app that is the same as 800x600 pixels.
I am using this plane to delineate a space in which to locate other objects.

So I have been reading the threads but just not getting it.
How can I determine what Units x Units dimension would equal 800 x 600 in pixels?

The Orthographic camera size setting affects the area the camera covers. It’s half of the screen height in units. So if your screen’s vertical resolution is 1050, setting orthographic size to 525 would result in 1 pixel == 1 world unit. 800x600 sprite with a “pixels to units” setting of 1 would appear 800x600 on screen.

If for some reason (physics scale) you have to use a smaller camera size, you have to scale up “pixels to units” with the same ratio.