How to select shunt resistor value of the below circuit? We can use Ohm's law but I am still a bit confused
A shunt resistor value is a trade-off between how many millivolts you can afford to lose, and how easy it is to measure the voltage. Common values for the voltage drop at the rated current are 50 mV or 100 mV. If you want 50 mV for 5 A, then the resistor would be 0.05V/5A = 0.01 ohms.

If you can afford to lose a higher voltage, then it will be easier to measure. If you want to lose a smaller voltage, it will be harder to measure.