What is the longest measurement in mathematics?

Answer 1

You can use any of the standard SI prefixes with the standard unit of length, the meter. However, it's more common to use non-SI units: astronomical units, light-years, and parsecs for large distances, especially in astronomy. The largest of these is the parsec; but you can also use metric prefixes with that, e.g., kiloparsec, or megaparsec.

Answer 2

There is no longest measurement as there can always be a longer measurement made.

Answer 3

There can be no longest because "one more than the answer" is always longer.