A metra is a unit of length. It has no real equivalent in English, and therefore cannot be translated by translator microbes. A metra is longer than a kilometer.
Metras are subdivided in motras, with 600 motras being slightly more than half of a metra.
Metras are most commonly used to measure the size of massive spacefaring vessels in the Uncharted Territories, like Command Carriers.
Source[]
Metras are taken from Farscape. For canon information, please visit the Farscape Encyclopedia Project.