You need to simulate the schematic above and determine what values you need for the series R and the capacitor. Then calculate power. But 25W will be huge, way larger than you need. If you need 100ma then (0.1A)^2 * 100R = 1W so you should use a 2W or larger on the safe side. More importantly you need to make sure the resistor has a working voltage greater than your line voltage. This is easy in 120VAC countries, but might be a concern in 240VAC countries.
That schematic is just one I pulled off the net. It's good for 20ma. If you want 100ma you need to change the main cap to 5uF and the filter capacitor needs to increase to 3300uF or more, depending on how clean you need the voltage.