If you had drunk milk since you were little, you would have been 180cm by now.
If you had been a milk drinker all your life, I bet you would have grown to be 180cm by now.
If you had drunk milk since you were little, you would have been 180cm by now.
「もし小さい頃から牛乳を飲んでいたら、今頃１８０cmだったかもよ。」
If you had been a milk drinker all your life, I bet you would have grown to be 180cm by now.
「もしずっと昔からミルクを飲む人だったら、今頃１８０cmに育ってただろうにね。」
どっちも If you had ..., you would have ...の形で「もし〜していたら〜だったろうに」という言い方です。
drink milk「ミルクを飲む」
since you were little「小さい頃から・子供の頃から」
by now「今頃には」
grow「成長する・伸びる」
ご参考まで！
カナダ 
