In Revelation 21:17, what is meant by the phrase, "measure of a man, that is, of an angel"?
"Then he measured its wall: one hundred and forty-four cubits, according to the measure of a man, that is, of an angel" (Revelation 21:17).
John is trying to give us the impression of the immense size of this place, so he tells us that it is 144 cubits, but in his day a cubit was not nearly as precise as we make measurements today. A cubit is the length from the elbow to the tip of middle finger. Generally it is about 18 inches, but we have found variations, which you would expect since everyone is not exactly the same size.
But this gives some potential misunderstand, after all it is an angel who is making the measurement. Some might argue that angels could be a different size than humans. So to emphasize it, John says the cubit is according to the measure of a man, but he also points out that an angel's cubit is the same length anyway.
By this, John is also making the subtle point that in heaven angels and men will be equals. "But those who are counted worthy to attain that age, and the resurrection from the dead, neither marry nor are given in marriage; nor can they die anymore, for they are equal to the angels and are sons of God, being sons of the resurrection" (Luke 20:35-36).