If the transistor is being used as a switch (in the common-emitter configuration) then you need to ensure the base gets enough current to saturate the transistor at the desired collector-current.
You also must ensure the output pin isn't providing more than about 25 or 30mA to protect it. Thus the minimum resistor value is 180 ohms.
If you want a collector current of 100mA, say, and your transistor has a (minimum) gain of 100, then it needs 1mA to conduct that current, but significantly more current to properly saturate, say 2 to 5mA. The base resistor will have about 5 - 0.7V = 4.3V across it, so 5mA means something like 820 ohms.
If the transistor isn't saturated its only "sort of" turned on and won't be a good switch.