"15 And he who is speaking with me had a golden reed, that he may measure the city, and its gates, and its wall;
16 and the city lieth square, and the length of it is as great as the breadth; and he did measure the city with the reed -- furlongs twelve thousand; the length, and the breadth, and the height, of it are equal;
17 and he measured its wall, an hundred forty-four cubits, the measure of a man, that is, of the messenger;" (YLT)
The "measure of a man" describes the length of the cubit... as a man would measure. Think of it as "yardstick" or "ruler." It is not discussing the height of a man nor the height of the angel.
Clarke's commentary on verse 17:
"The wall - a hundred and forty and four cubits - This is twelve, the number of the apostles, multiplied by itself: for twelve times twelve make one hundred and forty-four.
The measure of a man, that is, of the angel - The cubit, so called from cubitus, the elbow, is the measure from the tip of the elbow to the tip of the middle finger, and is generally reckoned at one foot and a half, or eighteen inches; though it appears, from some measurements at the pyramids of Egypt, that the cubit was, at least in some cases, twenty-one inches.
By the cubit of a man we may here understand the ordinary cubit, and that this was the angel's cubit who appeared in the form of a man. Or suppose we understand the height of the man as being here intended, and that this was the length of the measuring rod. Now allowing this height and rod to be six feet, and that this was intended to have some kind of symbolical reference to the twelve tribes, mentioned Revelation 21:12, represented by the twelve gates; and to the twelve apostles, represented by the twelve thresholds or foundations; then twenty-four, the number of the tribes and apostles, multiplied by six, make precisely the number one hundred and forty-four." Source: here