If you're basically being asked to do a load calculation, then time and materials. The load calculation will determine the size of the new service. not saying it is impossible, however I have never seen a home of that square footage that required more than a 400 / 320 amp 1ph service, tho a large on demand electric water heater and numerous HVAC equipment with electric backup heat could certainly do it.
as I found out the other day, homes can have this three-phase service for the original HVAC equipment, which uses just single phase for everything but the air conditioning, and would have an relatively small conductor (in relation to the other service conductors) coming off the highleg.
If you are doing the entire wiring project, I would check with the power company to see what they can or will supply and at what cost. even if your load comes out less than 200 amps single phase, at the very least the service drop and meter pan will also have to be changed out. I would also make sure the homeowner is not planning on using commercial grade 3 phase HVAC equipment as a) a new three-phase service may not be available or prohibitively expensive and b) even if it is available it will require 3-phase panel, which are going to be more money as well.