Sam would most likely spend his time at the size he’s used to. It’s been a very long time since he was human, and his knack aside, it’s jarring to be out and about with humans around. Dean or their close friends are one thing, but now he’ll be facing people that might be just like the Mangas family. There’s no way to know.
Dean would be thrilled, and now they’ve got an even better secret weapon. Sam can slip through the walls and get into places no human can go, and now he can switch back to full-size if he’s in trouble!