Since I cannot find the correct size of the magnet, I use another solution to solve the rusting of the magnet permanently, I pull out the floating unit which contain the rusted magnet and discard it, consider the rust may cause harm to your body as you drink the coffee which made with the water everyday. To let the sensor detect the water level is normal I stick a magnet directly on the box of the sensor which is located inside the back cover of the coffee machine, and the blinking problem is immediately solved. The pro and con of this solution is :
Pro: No more worry about the blinking issue and the harmful rusting water odor.
Con: Need to be aware of the low water level of the tank.
Hope this will help those who have the same problem of their Senseo coffee machine.