Re: How big are these reflective spots? And do we have an El-Reg unit of reflectivity?
The largest has a diameter of just over 10 km or an area of 69000 Olympic swimming pools.
Ceres' visible albedo is 0.090±0.0033, that is pitchblack. Use your favorite graphics editor to compare R/G/B=0/0/0 with R/G/B=23/23/23 to convince yourself. I do not know how white the "white" spots are.
Beer because Ceres was the goddess of grain crops.