The english is odd here - it would be ok if he is measuring a radio signal or the height of lamp posts along the street but if he is measuring the length of the street as i suspect then we would say he is measuring the length of the street.
There are many sentences in this course which, without any context, seem unfinished or "clipped". This one seems like that in both Romanian and English. What it means is that he measures SOMETHING along the street, but we don't know what. It would have been much clearer if this sentence included an object, so that it looked like "He is measuring THIS-OR-THAT along the street".
Anyway, if he were to measure the length of the street, then that would be "El măsoară lungimea străzii".
The point here is to learn that "de-a lungul" means "along" and requires the genetive case; the sense is "He measures the street longwise / lengthwise" as opposed to "across" (measures the length, not the width).