Your water is acidic and that requires an acid neutralizing filter. They add hardness to the water. Then you need a softener to remove the total hardness, iron and manganese. Some here will say you need an tron filter instead of using a softener to remove the iron etc.. They love spending your money for you and disagreeing with most things I say.
We need to know hw many people and bathrooms and if there are any large tubs, 2 person or body spray showers etc.. The sq ft of the house has nothing to do with this.