Big Boos are basically larger Boos. Big Boos can be seen alongside smaller Boos in Hammer Bro Hideout and Covenant Assult, where they guard exits, Covenant members, or Hammer Bros. They have similar attacks as a normal Boo, if you look at them, they cover their eyes, but turn around and it follows you! It can only be defeated by shells, guns, or a fully charged Megaman Mario shot.
One Big Boo is also the boss of Boo Mansion. To fight him, use the same strategy in Super Mario World: throw ice blocks at him! The main differences are that he takes more than three hits and he can summon smaller Boos in battle; no more than two Boos guard him at a time. He can't turn invisible either and follows a circular pattern.