As long it is well above the power supply which is 9 volts but people are known to accidentally hook up an incorrect power supply so IMHO a minimum of 18 to 30 volts. Other than that find the lowest profile. I suggest using MLCC type they are low profile, affordable and have a 5mm lead spread which is perfect.
They usually come in assorted values like below for a good price.
Be sure to check for the correct value range that you need.