My curiosity was peaked when looking at this application circuit from linear: http://cds.linear.com/image/Fig2_-_Wireless_Current_Sense_Circuit_Floats_with_Sense_Resistor.png They put two 2M resistors in series instead of one 4M resistor. I've never really used anything bigger than 470K so I was wondering why someone would do that. Especially as the change is just a factor of 2 and not an order of magnitude. That just seemed unusual to me. At first I assumed it was the parallel resistance of the FR4. I found online they guarantee 10^9 ohm/cm even when damp and hot. If I assume the R is 1mm, to keep the math easy, then that is 0.4% of 4Meg and of course 0.2% of 2Meg so in both cases one is not getting to 0.1% overall tolerance. Maybe it's a way of controlling shot noise (i.e. the probability of two large simultaneous events is much smaller) or it's something related to the manufacturing process? As part of searching for an answer I found the rule of thumb: Don't use anything larger than 1Meg. But there was not explanation as to why. Maybe this rule is the reason?