Zero Cross Detection Circuit for line voltage
You can get big resistors.

The datasheet specifies 1.15V - 1.5V forward voltage when the current is 10mA. To get that from 120V you would need a resistor of (120-1.5)/0.01 = 11.9kOhm, it would dissipate about 120*0.01 = 1.2W, so a 2W resistor would probably be OK (or use a 5W one if you have space, it won't get as hot). 

It would be difficult to get a resistor that precise, but the LED in the optocoupler can survive a continuous current of 60mA (which would require a resistor smaller than 2k), a 10kOhm 5W resistor in series would work. The current will be somewhat above 10mA, but nowhere near the maximum.