A measuring rod is the length and stature of a man--meaning John was told to measure or count the number of people of the house of Israel up until the time of the cross, who make up that inner portion of the spiritual Temple in heaven.
The outer court of the gentiles--meaning the saved of the gentiles, were not to be counted at that time. But would "tread" (meaning stand upon) the foundation laid by Israel, during the times to come after the cross.
Those are the two witnesses. As it is written, "there are three that bear witness on earth: the Spirit, the water, and the blood; and these three agree as one" (1 John 5:8). "The water" refers to those "born of water" as Christ explained, meaning of flesh, after the likeness of Adam. "Blood" refers to those born of the saving blood of Christ, meaning "born again of the spirit of God." "The Spirit" is God.
All of which is rightly referred to as a riddle by the confusion placed upon all language at the Tower of Babel.
Again, these are the two witnesses of God.
As for "fire" proceeding from their mouth--this is the Holy Spirit speaking the truth from God "precept upon precept, line upon line, here a little there a little" (Isaiah 28:10) through God's people down through the ages, resulting in the scriptures as well as every good word from God unwritten, but according to His purpose. Which yes, is the death of the enemies of God.
As for "the power to shut heaven"--this is the power to believe and push forward the plan of God, or to stifle the plan of God by unbelieve...just as it is written of the good and evil kings of Israel--they were a foreshadowing of what is now true in the spirit. In other words, specifically after the cross, and even before, "we do not wrestle against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this age, against spiritual hosts of wickedness in the heavenly places" (Ephesians 6:12).