How do you determine how large the utility supply will need to be in order to support the demands of the service?

When designing applications, you rely on standards. The National Electric Code dictates the size of electrical components, States usually regulate the required sizes for water supplies and wastewater treatment.

If you are trying to determine what the standards should be, then you typically make some assumptions and do some testing to determine what would create a resonably reliable system using standard engineering practices and available technology.