If the current consumed by the resistor is a problem, use a large value (say 1000k). If this is not a problem, use a small value (e.g., 2k2).
A large resistor value reduces current consumption, but it may be sensitive to noise, or phenomena such as POST#33 may appear. A small resistor value will increase unnecessary current consumption.
The maximum practical resistor value is 1 MΩ. To improve the accuracy of AD conversion, the voltage divider resistor should be as small as possible. And finally, the resistor value is determined by your required specifications.