Hallo J.

if you apply 110VDC to the circuit of Michael, only about 10mA is flowing through LED of optocoupler 4N35. From datasheet of 4N35 you can see, that 10mA causes a voltage drop of 1.2V typically across LED of optocoupler. So, 110V - 1.2V gives 108.8V. This voltage drops across the two resistors 11k and 100R, resulting in 108.8V / (11k + 100R) = 9.8mA.
9.8mA times 100R is 0.98. Plus 1.2V of LED of optocoupler makes 2.18V. This voltage lies also at 4.7V zenerdiode. But because 2.18V is much smaller than 4.7V threshold voltage of zenerdiode, NO RELEVANT current flows through zenerdiode.
Situation changes, when overvoltage is present at input of this circuitry. Then, zenerdiode limits rise of voltage across LED-100R-combination to less than about 4.7V. Then, and only then, 34mA is flowing through LED of optocoupler. From datasheet of 4N35 you can see that 1.3V drops accross LED when 34mA are flowing. 34mA times 100R makes 3.4V. Plus 1.3V of LED gives 4.7V.
Et voilà, mon capitaine...

Datasheet of 4N35 tells, that continuous LED forward current may be 60mA. So, with 34mA evrything is fine. 34mA makes a power dissipation across 100R resistor of 34mA x 34mA x 100R = 0.12W, so, the 100R resistor isn't overloaded, either.

When voltage at zener diode rises up to 4.7V 34mA are flowing through 11k resistor. This makes a voltage drop of 374V. So, voltage at zenerdiode rises up to 4.7V, when at the whole circuit 374V + 4.7V are dropping. If voltage rises further, also through zenerdiode current begins to flow. This current adds to that through the LED-100R-combination, means 34mA. If you use a 500mW zenerdiode, up to 106mA may flow through zenerdiode. Then, 106mA plus 34mA are flowing through 11k resistor, which makes a voltage drop of 1540V! So, even if overvoltage is rising up to about 1550V, no damage is to be expected for optocoupler, 100R resistor and zenerdiode. Only 11k resistor must be able to withstand this high voltage. But, don't be afraid, overvoltage along your 110V line should be much smaller, shouldn't it?

Nevertheless, 11k resistor should be a high power type. Under no-overload-condition 9.8mA is flowing through this resistor, which makes 1.06W. So, choose at least 2W. During overload condition, current flowing is much higher, but don't forget, that overload condition lasts only a very short time. So, in most cases, 2W type should be sufficient. But if you observe abnormal heating, then increase power rating, of course. Make an analysis of overload condition and decide for yourself, whether a 2W type is sufficent, or not.

Kai

PS. Please don't write "b'coz"! Or what would you think if I wrote "§&$%(}" for "Volt"?