noun which may take the genitive case particle 'no'
long-legged; having long legs
Describes a person, animal, or thing with notably long legs. Often used attributively with の.
彼女は足長で、モデルのようだ。
She is long-legged and looks like a model.
あしながグモは細長い脚を持つ。
The daddy longlegs spider has slender legs.
長身 means 'tall stature' in general, while 足長 specifically refers to having long legs.
Compound of 足 (あし, 'foot, leg') and 長 (なが, 'long'). The reading なが is a rendaku form of 長い (ながい).