What ever the value ( a practical value ) of R2 you use, you will always measure no voltage across it, because R1 is so high then virtually no current flows through it.
If you're considering something like that, go back to the drawing board.
As others have pointed out your concepts are just wrong.
The input resistance, is, for all practical purposes, infinite unless you use the INPUT_PULLUP instruction.
A little bit harsh there, don't you think?
My example might not seem practical to you. However, if we're talking about an electrical circuit (the one from my schematic above) which I could easily build on my desk right now - I consider this to be very practical. And if you will use a ≫1 MΩ resistor it will still behave in a certain way. It might not be what the overwhelming majority is concerned about, but I still like to think about what if. I really dislike the generalizing statement "never need to know the exact value for the resistance" - even if there isn't an exact value for this to begin with.
And if you will use a ≫1 MΩ resistor it will still behave in a certain way.