。
She has a slender build.
Literal
She [topic-は] slender build is.
華奢 (きゃしゃ) is a fairly literary na-adjective meaning 'slender, delicate, slight of build' — applied to people it describes a fine-boned, fragile-looking figure. The kanji combine 華 ('flower, splendor') and 奢 ('luxury') for an image of refined delicacy. 体格 specifically means body build or physique. The sentence is short and structurally simple — topic + na-adjective phrase + copula — but the vocabulary is on the bookish side; you'd encounter this in a description in writing more often than in casual speech.