resistor value in wheatstone bridge
Ideally a Wheatstone bridge is balanced. Where you have one sense resistor, this means choosing the other three resistors to be the same as its mid range value.

Under certain circumstances, you may want to use lower value in the reference arm for slightly lower noise, or lower values to ground to offset the common mode from mid scale, or higher values to reduce current consumption and/or heating. But usually, all balanced is the place to start.

In your case with a sensor that ranges from 0.7 to 1.8Mohms, the geometric mean of those at 1.1 or 1.2Momhs would be ideal, but 1M would work quite well with little increase in the required common mode range.