If your resistor is too small, then the photoresistor will not be able to "pull down" the voltage, and your readings will mostly be in the higher range of the voltage spectrum. If the resistor is too large, then the photoresistor will pull down the voltage too easily, and the readings will mostly be in the lower range of the voltage spectrum. This may or may not be a problem depending on how good resolution your ADC input has that reads the voltage. (A typical AVR with 8-10 bits would prefer a well matched range.)
The formula on the page gives a resistor value that gives Vout approximately at half when in a mid-amount of light.