The wattage, W, being dissipated in the resistor is given by...

W = I²R = 0.02² * 470 = 4E-4 * 470 = 0.188 watts

The common standard power ratings of resistors are 0.25W, 0.5W, 1W, 2W, 5W, and 25W so even the smallest size can be used in this application.