Indeed, it has to be a small one, maybe a young from last season. Once Spring comes around they put on some weight and shouldn't be able to fit through a 25mm hole.
We've had sparrows breed in everything you can think of. My parents had those ceramic flowerpots with a hole in the bottom, hanging with the top against the brick wall of the house.
Maybe you can put up another box or two with different sized holes. House sparrows are just interesting to watch as any other bird.