They do not use units: they use the fact that stars are not galaxies. For example, you don't use units to measure the difference between children and countries.
It is possible that someday astronomers will measure all the distances of the 100 billion galaxies in the universe.
It is possible that astronomers will measure all the sizes of 100 billion galaxies in the observable universe.
Someday astronomers may have measured all the distances of 100 billion galaxies in the observable universe.
No. We do not have time to make so many measurements.
The parsec is 3.26 light years. Astronomers measure distances to remote galaxies in megaparsecs--millions of parsecs. This is about the longest commonly used length metric.
How do they measure what?
difference in differences uses panel data to measure the differences
The general tendency is for galaxies that are farther away from us, to move away from us faster. This can give a rough idea of a galaxy's distance, just by observing its redshift (which is a measure of how fast it moves away from us).
lightyears
Parallax is the method that astronomers use to measure the distance from the sun to the earth.
difference in differences uses panel data to measure the differences
Astronomers use the unit of measure "Light Years" to calculate the distance between pretty much anything in the Universe.