Start thinking Heat Transfer instead of electricity.
1 - What is the object you are trying to raise from ambient to 600.C? How much heat will the object absorb? How quickly does it have to happen?
2 - What are the heat losses through the walls of your furnace. It is open face, or can you close the door?
3 - I usually go back to Heat Transfer textbooks to check up the fundamentals, and to find heat transfer coefficients.
4 - When all else fails look around the market place for a similar furnace and check what kW rating they use.