If you go by height, she is standing a little bit before the middle of the line.
By the order of their height, she is the third or the forth from the beginning of the line.
ご質問ありがとうございます。
① "If you go by height,"＝「背の高さの順番だと」
"she is standing a little bit before the middle of the line."＝「彼女は列の真ん中より少し前に立っています。」
② "By the order of their height,"＝「背の高さの順番で」
"she is the third or the forth from the beginning of the line."＝「列の始めから3番目か4番目です。」
ご参考に。
アメリカ合衆国 
