Page 3 of the data sheet says the the typical output range of pin 1 is 10uA to 500uA. This also means you can probably get a dynamic measurement range of 50:1. Because 500uA is the maximum current, you'd want to choose a resistor that doesn't cause the output to hit the 5V end-stops so if you reckoned 2.5V is the maximum voltage output when 500uA was being driven from pin 1, this equates to a load resistor value of 5kohm.
On the other hand, if you chose the circuit of figure 19 - the current output from pin 1 goes into a virtual earth op-amp circuit and therefore the restriction on impedance is of no importance any more - but obviously you have to obey the rule of not having so much gain in your op-amp circuit that the output hits the rails.
I think it's worth mentioning that although when operating as a voltage to frequency converter, a dynamic range of 100dB is attainable (1Hz to 100kHz) you won't be able to get anything like this when operating as a frequency to voltage converter (as mentioned above).