What size isolation resistor to use in CC'd network??

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I've read on this forum that a resistor ranging from 2MegOhm-5MegOhm can be used at the midpoint of biased caps. My question is, can I just arbitrarily choose a resistor value (2M-5Mohm) or does further consideration such as cap size, voltage of cap, type of cap determine whether a 2M, 3M,4M or 5M should be used??

Thanks,

Lamont