The PLECS synchronous machine models don’t have a concept of “rated power”. Machines of different power ratings would simply have different electrical and mechanical characteristics. These will manifest as different resistances and inductances in the equivalent electrical circuit, and different inertia/friction values in the mechanical domain.
Do you have some data representative of a machine you’d like to model in PLECS?
Once you have the characteristics of a representative machine, reach out to Plexim support (firstname.lastname@example.org) if you need help translating the data into the required format for the PLECS machine models.