。
She has a slender figure.
Literal
She [topic-は] slenderly figure [object-を] is-doing.
The pattern ~した姿をしている (literally 'do an X figure/appearance') is parallel to ~顔をしている — Japanese uses する to predicate physical features as 'wearing' or 'embodying' an appearance, where English would say 'has.' ほっそり is a state mimetic for slim, slender, or willowy build — with した attached, it works as a noun-modifying past form ('having become slender'). Common in body and character description.