She pulled down her hair its full length.

完整的句子应该是:
she pulled down her hair and let it fall to its full length.
你要写成
She pulled down her hair to its full length. 也OK.
"2.全身伸展地,全身平伸地；直挺挺地[亦作at one's full length ] " 是对的, 但不适用在这一句, 因为这一句在 "full length" 已有了一个动词, 不可能接at的
这一句就OK: He was at full length after ho took a nap. ( 他在小睡片刻後全身舒展.)
以上, 希望能帮到你~ ＾0＾