how to calculate the value of current limiting resistor connected to zener on full load conditions?
If the load isn't attached then the resistor can be massively bigger and, in these circumstances, you only need to feed a few milli amps to the zener to keep a regulated supply BUT, if you then connected the load you would find that things go wrong and the output voltage would fall to some lower-than-acceptable value.



Basically, "IL" is the load current plus the zener bias current (a few milliamps but read the data sheet for the zener diode).

So, if your output voltage was meant to be 5.6 volts and your load might be as low as 100 ohms, the total current (worst case) thru Rs is 56mA plus (say) 5mA for the zener. This current of 61mA flows thru Rs and if the input voltage (Vs) was 12 volts then Rs = (12-5.6)V/0.061A = 104.9 ohms (maximum). It has to be a maximum because if it were any higher, under full load conditions the zener would not be in regulation because the mid-point voltage of Rs and Rload would be below the zener voltage.