I wonder if it's not a mistake to say :''Her feet/hands/legs/waist/body are slim instead of saying : ''She is slim'' ?
I would not recommend: slim feet, slim hands, or slim legs.
However, I would recommend: slim waist and slim body.
Being slim is something desirable in many cultures. Being thin or skinny sometimes suggests the result of an illness, hunger, malnutrition, etc. The terms are not always negative.
She looked very thin after the surgery. (This sounds negative to me.)
He used to be chubby, but now he is skinny. (He used to weigh too much, not he does not weigh enough.)
Among friends you may hear "How do you stay so skinny?" It is said with humor and is not meant to be insulting or offensive. On the contrary, it is a form of flattery.