First to answer your question - you calculate it from the voltage across the resistor - P = V*V/R where P is in watts, V is in volts, R is in ohms or P = I * I * R where I is in amperes and R is in ohms.

Now having said that the currents in the tutorial you are working with are so small that any of the resistors you found (1/2 W, 1/4 W even 1/8 W) will work just fine.