Hi Miles,
This is going to depend on the input bias current and whether you are bypassing the PGA. There are graphs showing the input currents in the Typical Characteristics graphs in the datasheet. You may notice there are differences with respect to the inputs so matching the input channel characteristics is important. AIN0 and AIN1 is the best combination.
I generally target around 1 to 2k resistors for general operation. The larger the value the more care is required. For example, if you use large resistors they should be well matched with low drift characteristics (in other words precision resistors) and then error can be calibrated out of the system. I would stay below 10k if at all possible.
So what are your design parameters with respect to maximum input voltage, gain, etc.?
Best regards,
Bob B