Why is the man going to measure Jerusalem in Zechariah 2:2?

In sensus plenior, measuring is a type of judgement. Jerusalem is measured in two directions, representing a judgement of it's spiritual works and it's earthly works. The man is Christ, to whom all judgement has been given.

   4 And said unto him, Run, speak to this young man, saying, Jerusalem shall be inhabited as towns without walls for the multitude of men and cattle therein:

Since cities are the inheritance of the Priests. Towns without walls are those without an inheritance in the priestly kingdom. This is a pronouncement, that Jerusalem has failed to be the spiritual and holy people God has called them to be and they are to be cut off.

There are two reasons for cutting them off:

There are lots of Adams and lots of beasts

Adams Adam is the same word used for the red which is the color of sin:

   Isa 1:18 Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool.

They are being cut off for their multitude of sins like Adam's where he rebelled, knowing that he was sinning.

Beasts

As shown elsewhere, the nature of original sin is in placing the instinctive nature that we share with beasts (the flesh) before the word of God. The invective that Jerusalem is filled with beasts is another condemnation of their sins of the flesh, like Eve's.

The lock

The word for breadth is Rachab which is also the name of the prostitute. The sin of the prostitute is a rebellion against God being a symbol of idolatry.

The word for length refers to a physical (earthly) measurement of distance or time. The sins of the flesh are sins of deception as was Eve's. God is more tolerant of these sins. And the word for length also means patient. God was patient with Eve's sin, to the point that he did not condemn the world because of her. Sin came into the world because of Adam's rebellion.