Q:

Power dissipation in triac driver optocoupler

This is a typical circuit for opto couplers:

What should be the appropriate wattage for 180 ohms resistor? 
I think power dissipation should be minimal because in off state, there won't be any current and in on state, the voltage drop across resistor will be very small.  As such is it advisable to use 0603 package resistor?  
Power rating for 0603 was given as 1/16 watts - 1/10 watts in various articles. 
Right now I am using 1206 package and it's working perfectly. 

A:

Then that is fine.  The resistor only conducts for a short period of time at start of each AC half cycle when the phototriac is on.  And only until there is enough current flowing through it to turn on the main triac.  The number determining the dissipation is the turn on current at the triac gate and as it is working fine this is low enough not to cause dissipation problems for the resistor.