this river is about 50 metres wide .the street is 5 kilometres in length.如果像第一句那样的表达,

那就只能改成the street is 5 kilometres long.//如果length不改就不能去掉in哦，那 long和length有什么区别吗?long在这里是形容词，而length是名词，需要和介词in构成介宾短语。