It depends on what you're aim is with the resistance / impedance in the earth-path.
Do you want to protect you're equipment with smaller earth-fault currents? (Low resistance earthing) Do you want to reduce step and touch potentials? (High resistance earthing) Do you want to obtain a earth-point in a delta-system? (Earthing through a zig-zag trsf)
Combinations?
You have to decide how much ground fault current you want to allow and for how long.
For resistance earthed systems, the resistance of the resistors determine the ground-fault current. You'll need to specify the resistance along with the duration how long they have to handle the current.
In a solidly-grounded system through a transformer (zig-zag trsf), the grounding transformer impedance largely determines the ground fault current. You will need to specify the grounding transformer impedance (normally in ohms) as well as how long the transformer must be able to handle this current (normally 10 seconds).
In a combination of a zig-zag trsf and resistors the total resistance (impedance of trsf and resistance of resistors) will determine the ground-fault current. You'll have to specify both + the duration of the fault current.
You have to specify:
Operating voltage / BIL
Desired impedance / resistance in ohms
Max. short circuit current
Duration of Short circuit current
Max. continuous current
Circuit voltage, duty and location in the electrical system are not specified, but for ANSI regions there is Chapter 1 {§1.4.3+} in the IEEE green book, {Std 142-1991} that has some basic considerations and references on forms of resistance grounding.
