The point is not that people and angels are the same size—as if 600 human feet (one stadium) is the same length as 600 angelic feet. Rather, John would want his readers to know that they are not to convert the numbers in this paragraph to something more humanly sensical (who ever heard of a city of 12,000 stadia or some 2220 miles!). Rather, the human numbers (12,000 and 144) embody a heavenly message. Here is a reference to the symbolic nature of so much of the book of Revelation, a symbolism that requires the reader to be careful to hear what the Spirit says to the churches.
17 He also measured its wall, 144 cubits by human measurement, which is also an angel’s measurement.