I'm currently studying a horse-leech, and would like to know if it is fully grown or not. When I read about different annelid species a maximum length is often given, so I wonder if there is some standard way of measuring these elastic animals? 

For example, my leech varies from 2 to 10 cm, but often seems to prefer to stretch out to about 6 cm when not moving.  

(Possibly I should modify this question to be just about leeches, but I also wonder about earthworms, and probably other soft and difficult worms as well...)