Concrete spec's for a wall 12' high?

You need to speak to structural engineer in your area. There is more to this than wall thickness and rebar spacing. Depending on where this house is built you will also need to design the house for lateral forces, such as wind and earthquakes. Since you are using concrete walls, the connection between the roof framing needs to anchored to the walls. Concrete is heavy and depending on the wall thickness your footing sizes will be increased. Depending on your local codes you may need to get an inspector to inspect the concrete walls.