His/her legs are so long that they are more than half of his/her height.
His legs make up more than half of his height.
ご質問ありがとうございます。
① "His/her legs are so long"＝「彼・彼女の脚はとても長い」
"that they are more than half of his/her height."＝「彼・彼女の身長の半分以上の長さである。」
② "His legs make up more than half of his height."＝「彼の身長の半分以上は脚です。」
追加：「羨ましい！」は "I am so jealous!" と言えます。
ご参考に。
Her legs are long. They cover more than half her body.
She has long legs. They go more than half way up her body.
ーHer legs are long. They cover more than half her body.
「彼女の足長いね。体の半分以上が足だよ」
to cover more than half one's body で「体の半分以上を覆う」と言えます。
ーShe has long legs. They go more than half way up her body.
「彼女足が長いね。体の半分より上まであるよ」
to go more than half way up one's body で「体の半分以上まで伸びている」のようにも表現できます。
ご参考まで！
アメリカ合衆国 
カナダ 
