Something "just for the knowing," is that even smaller resistors of the same ratio would also work. I doubt that you're going for "conversion speed" with this application, but the higher the resistor values, the longer it takes for the analog converter inside the arduino to do the conversion, since it involves charging a cap, and then timing the discharge (simplified), and the higher the resistance, the longer it takes for the cap to charge. High enough and the conversion can become distorted. Here's something to read.