Zero Crossing Circuit Input Resistor
Only you know the intent of the circuit, but I would say that for a dimmer you want to detect close to the actual zero crossing, say within 10V or so. 

So with input voltage of 10V you need output current of about 300uA. Since CTR is minimum 0.20, that means an input current of 1.5mA. So a resistance of 10/0.0015 = 6.7K (total). Power dissipation would be about 7W at 220V. That's very high. Even at 100VAC it's still 1.5W total for the two resistors. 

So I suggest you find a better optocoupler that responds reasonably quickly (not a darlington type) and has a CTR of 100% or more.