400 sq. feet would accommodate 3 10x10 rooms and a good sized bathroom almost as large. Add in a couple of sleeping lofts.
Our family cabin is about 16’ x 16’ I would guess (256 sq. feet). Two queen beds and a bunk bed make up one have. The kitchen, eating table and a couple small sofas, chair and pot-belly stove make up the other half. Hand pump out in the back.
Would spend all summer up there with my mom, three siblings, gramma and grandpa, and dad on long weekends. It was fine.
Oh. Add in another 8 square feet for the outhouse!