Wire sizes are set in the DM Electrical->Customization->Wire Sizing commands. There is a list of wire sizes in amps at the bottom of the dialog. These sizes are where you specify the wire size.
The wire size is used to size the phase and neutral wire.
The X (reactance) and R (resistance) values used to calculate the impedance for the wire should be taken from NEC Table 9.
There are two ground wire settings. The first is for sizing the ground wire on branch circuits. The second is for sizing the ground wire on feeders.