中等身材用英语可以说作 "average build"。这个说法来源于人体的身材可以用来描述人体的体型，而 "average" 指的是一个平均值，所以 "average build" 就指的是一个人的身材是比较标准的，不偏胖也不偏瘦。例如：

She has an average build, neither tall nor short.

He has an average build and is of average height.

She is of average build and weight, with a proportionate body.

He is of average build, with a medium frame and average muscle mass.