The value of the $$R_s$$ will depend on the power rating of the Zener diode to be used. If $$dfrac{6v}{0.5w}$$ is used , the maximum safe current thru Zener will be $$dfrac{[0.5] }{6}$$ = 83 mA Then minimum value of Rs will be $$dfrac{10-6}{0.083}$$ =  =48 ohms . You may use a standard resistor value 0f 51 ohm or 56 ohm. (If you want to reduce the power dissipation in Zener diode, 120 ohms may be used which will limit the current to 33mA. (29mA thru Zener & 4mA thru Load Resistance (1.5K).