The quality of being large and having a lot of space inside.
Spaciousness is usually good because it means there is a lot of room for people or things.
Examples:
The spaciousness of the room made it easy to play.
I like the spaciousness of the park.
The spaciousness of the car made the long trip comfortable.
The spaciousness of the beach made it easy to find a spot to sit.
The spaciousness of the universe is beyond our understanding.
The spaciousness of the field was perfect for running and playing.