In scientific papers, either measure may be used. Sometimes they use total length (includes the tail) and sometimes snout-vent length (excludes the tail). For hobbyists, it's generally total length that is used.
Total length can sometimes be a little deceptive. I have a male and female Cynops e. ensicauda that have the same total length. But the female is younger and much smaller, because females of this species have longer tails in proportion to their size. In this case, snout-vent length would be a better comparison.