External pull down device required, How much?
The exact value may not be critical. Typical ballpark values range from 1k to 100k. 

With a 1k resistor, lots of current flows through so it is considered "strong" or more immune to noise. This is at the expense of efficiency. 

With a 100k resistor, little current flows through so it is considered "weak" or more susceptible to noise. In this case efficiency is improved.

If the device is used in a noisy environment you would typically favor a lower value pull down but if you have a low power battery operated device you would favor a higher value pull down resistor.

10k is a very commonly used value since its easily available and balances both of these concerns.