The spacial was the unit of measurement used in the vast majority of instances, by both rebel and Federation crews.
Establishing the actual size of a single spacial is almost impossible due to the inconsistency of sources. On most occasions detector ranges were in the millions of spacials, but sometimes ships would only register within a thousand spacials' distance.
The Liberator was on at least one occasion visible from the surface of the planet it was orbiting, suggesting standard orbit was relatively low. By inference this would mean a relatively 'short spacial'. However, this in turn would mean detector ranges would be too short to make them effectively usable on a ship travelling at faster-than-light speeds, as the crew would only have fractions of seconds to react to anything showing up on the detectors before passing it.
Assuming a 'long' spacial, and giving detectors a much greater effective range, would mean that the Liberator usually orbited much further out from a planet than was usually implied (and would almost certainly have been invisible from the planet's surface).
It is possible that spacials were not an absolute unit of measurement, solving this problem, but how and in what circumstances they varied is completely unknown.
The only other unit of measurement referred to was the subsec, used in a single instance by the crew of the London.