Power rating for opto-triac trigger circuit resistors
The resistors on the optotriac carry essentially no current at all, only the brief pulse to trigger the triac. after that the triac carries all the current. use 0.25W or whatever is convenient, use resistors rated for 400V or more.

The 39 ohm resistor carries about 0.3mA while the lamp is off.  I-squared-R puts the power at less than 5uW  if you're using phase control peak currents could be higher resulting in more power consumption but quarter watt should be fine there too.