Voltage Divider Circuit
Choose what precision you want. Lets say .1V(100mV).

Now you look up your op-amps leakage current. Lets say it is 1.5uA.

For a voltage drop of .01(10mV) from 1.5 uA(.0015mA) you need a resistance of 10kOhms.

If I stay below 10kOhms I cannot get more than a 10mV voltage drop from it. You now know what you upper limit is for the resistance of your divider.

If you need higher precision, use an op-amp with lower leakage current or use a lower resistance. In all reality with a 1.5uA leakage you will probably not know the difference with a 100K resistance, but I always plan for worst case and then test it.

Let me know if there is anything I can do to make this more clear.