Best resistor value in series with LDR sensors
To get the largest voltage swing for the values you mention, R1 should be ~ 100 kOhm. But note that this makes the impedance of the Vout ~ 50 KOhm. For PIC UCs a maximum resistance of 10k is recommended, I don't know this figure for your UC *uno = Atmega328P?) so you'll have to check.

Also, I am not sure why you place the LDRs in series, but that's another question.