Difficult concept to wrestle with, isn't it?
It might help if we took a step beck up the text, a bit to pick up some additional info.
“And he that walked with me had a golden reed to measure the city withal...and he measured the city with the reed, twelve thousand furlongs...and he measured the wall thereof, an hundred and forty four cubits, by the measure of man, that is, of the Angel.” — Rev. 21:15-17 1599GEN
We don’t find out how long this reed is, but the angel measures some things with it, and John gives us the numbers along with this statement “by the measure of a man” to emphasize that the units given are the same ones his readers would be familiar with, which is further supported by the appended “that is, of the Angel,” which draws an equality between “the measure of a man” and that “of the Angel.”
IOW, a cubit’s a cubit’s a cubit.
We don’t know how many cubits high the angel is, but John doesn’t comment on his size, so it doesn’t seem that the angel would be either inordinately large or small. Elsewhere, when things are of unusually great or small size, John gives appropriate descriptors, so I’d think it an unusual omission for John not to comment if the angel had been of some fantastic size.