The size of a barndominium should mainly depend on your needs. If you are a family of 4 or 5 people, then you can go for a bigger house, and if there are just 1 or 2 people living in the house then a small house would also work. However, some of the common house sizes for a barndominium are 40x60 (2400 square feet) and 40x30 (1200 square feet).