For the resistor value, gauge it to the amount of current going through the network. In low power circuits reasonable R= 1-100 kohm.
The capacitor needs a certain amount of current to 'operate' it, that is, to generate desired voltage swings. Large C needs greater current to achieve intended performance. Small C is associated with small current.
A given RC integrator (low-pass) acts within a particular frequency spectrum. Above that range you get practically no output.
Below that range your signal is not diminished.
Often you must experiment with values to make an RC network that works in your intended range of frequencies. The time constant (R times C) provides a guideline to making calculations.