How do I determine the capacitor to use when converting current to voltage?
The capacitor determines how fast the circuit responds to changes in illumination.

The value 0.5 (which should actually have units of seconds) might have been appropriate for some particular application that the person who wrote the datasheet had experience with, or for what they were considering as the "typical" application when they wrote the datasheet. 

You should choose the capacitance to fit the response time you need in your application. 

If there were to be a step change in illumination (for example, the light is turned on in a dark room), the circuit will respond with an exponential curve toward the final value given by \$I_{ph}R_L\$ with a time constant given by \$R_LC_L\$.