Load resistor value for phototransistor switch
A larger resistor means that there will be more margin to assure operation if the opto ages, the optical path gets dirty (for interruptors) or the temperature changes. If you go nuts with a really high value you might have to consider dark current and leakage. 

It also means that turn-off will be slower- see this graph in the LTR-301 datasheet: 



As you can see the turn-off time may be in the 1.5 millisecond range with a 100K load.