[BrianG]

From the "No Spark" page at my site:

For most ESC's with around 1,000uF of total capacitance, use a 1Ω 2watt resistor when using up tp 25v (6s lipo), or 10Ω 5watt resistor when using voltages over 25v. This value will charge the capacitors slow enough to avoid the hookup spark, yet fast enough to still allow the ESC to arm properly (under half a second). The power rating is low because the only time the resistor has to dissipate the maximum power is on initial hookup. After that point, the differential voltage between the supply and the capacitor charge decreases quickly, which reduces current and therefore power dissipation.

This is a general rule of thumb. If you are using much higher capacitance, odd voltages, etc, the calculator at the bottom of the page lets you enter specific values and then shows the results. The calc does not show the specific power value to use, but generally, average the power values for the times shown.