The height of something, especially a person or a building.
Tallness can be good, like when someone is tall and can reach things, or when a building is tall and looks impressive.
Examples:
Her tallness makes her stand out in a crowd.
The tallness of the tree makes it hard to climb.
The tallness of the giraffe helps it reach the leaves on the trees.
The tallness of the building makes it look like it touches the sky.
The tallness of the mountain is so big, it looks like it can touch the clouds.
The tallness of the basketball player makes it easy for him to score.