in order to determine the power rating you'll need for a resistor, look at the absolute maximum voltage and current that will be going through it. Then it is usually a good idea to choose a resistor rated for at least twice this value, which will handle fluctuations--not to mention the fact that the voltage is probably measured in RMS, so you will have peaks to worry about.
The power rating of the resistor is not critical provided it is high enough to handle the maximum power that it might have to dissipate. When in doubt, go for a higher wattage.