The state or quality of being slender, or thin and graceful.
Slendernesses is usually seen as a positive thing, like when someone is graceful and elegant.
Examples:
Her slendernesses made her a beautiful dancer.
The slendernesses of the tree branches made them sway in the wind.
I admire the slendernesses of the ballerina's body.
The slendernesses of the cat's body allowed it to slip through the narrow opening.
The slendernesses of the spider's legs were almost scary.
The slendernesses of the model's figure made her stand out on the runway.