How small should I make my shunt resistor?
The solar cell's current won't change much if it is driving into 0 V or 0.5 V in your case, so, if you expect 20 mA max, you could use a R of 0.5V/20mA = 25 ohm. These are not easily available -- use 22 ohm. Total power is 12 mW, so any resistor would work.

With 0.5 V signal, you can convert directly with your ADC; you don't need an amplifier.