The extra wire in the attic is not causing your heater to consume more energy.
No you can't use a SP switch to kill power to the water heater.
Use a non fused, 2 pole, quick disconnect, if you want something to kill power to the water heater without shutting the breaker. It can be located near the heater.
The wire size determines the amperage of the breaker. 12/2 WG would use a 20A breaker 10/2 WG would take a 30A breaker 120V would use a SP breaker and 240V would use a DP breaker.
The watt draw of the heater, which is based on the wattage of the elements, would determine the wire size needed.