"Not tall and not overweight" 可以用英文表达为 "of average height and weight"。
关于这个表达的来源，"average" 这个词本身就有平均的意思，所以 "of average height" 就表示身高在一般范围之内，"of average weight" 就表示体重在一般范围之内。
下面是一些例句，希望能帮到你：
She is of average height and weight. 她个子不高不胖。
He's not very tall, but he's not short either. He's of average height. 他个子不是很高，但也不矮，算是个子正常。
She's not skinny, but she's not overweight either. She's of average weight. 她身材不瘦，但也不胖，身材正常。