Largest Rsense possible for BQ27510

The main criteria for your sense resistor is that the peak current does not create a voltage across it that is larger than the allowable input for the ADC input (VSS+/-0.125V).

You don't need to worry about the voltage across the sense resistor during your idle periods because the gauge doesn't monitor current during those times.  It sounds like you expect to read current values (<1mA) from the gauge during idle periods, but it doesn't work that way.  The gauge actually goes to sleep during these times to save your battery, and even if it tried to measure the currents it wouldn't be very accurate, as you can imagine.  The main purpose of the current measurements is to integrate them and count the coulombs flowing in and out of the battery to track the capacity level.  It uses this coulomb counting in conjunction with open circuit voltage (OCV) measurements when the current is low to figure out the state of charge.  There is a deadband setting in the dataflash (typically 3mA) below which the gauge will consider the current to be 0.  If there was no deadband then any offsets or noise in your PCB could cause a spurious or continuous slow counting of coulombs which would trick the gauge into thinking the battery was slowly charging or discharging.  Eventually you would end up with a vastly wrong reading.

A more detailed treatment of this topic is in SLUA439 on page 9.  Although the document is for the bq2750x, it still applies to all our handheld IT gauges.