According to Taube (1966)$^1$:
The adults of American leeches range from about 1/4 inch to 12 inches in contracted length.
Because leeches can bloat to more than 10x their "normal" size after a meal (and can undergo up to a 300% change in length), it seems that measuring a leech at anything but their contracted state would lead to wildly too variable of measurements. Therefore, it seems likely that only contracted size would prove to be truly useful.
However, see Klemm (1982)$^2$ (or Klemm,1995 $^3$) for comments about preferred preservation for identification purposes:
[Leeches] have a soft, highly contractile body. Therefore, if leeches are dropped alive into preservatives such as 70% alcohol (ethanol)...they contract strongly and sometimes such features as the eyes, general body shape, and the genital pores become distorted and difficult for the non-specialist and occasionally even for the expert to discern. When leeches are properly anesthetized prior to fixation, there is usually less muscular contraction. the body shape remains more uniform, and there is less variability of the anatomical structures. ... The preserved leech should be straight, moderately extended and undistorted.
- However, Klemm does not use body length as an identification feature in his key, and therefore his directions seem to be more concerned with preserving other anatomical features (not to ensure accurate length measurements). He does provide "average size" for species in his key but notes that "contraction during processing" can lead to variation in measured length.
Interestingly, naturewatch.ca provides the followign instructions for measuring earth worms:
1.Allow the worm to freely extend itself as if it was crawling.
2.Measure the maximum distance the earthworm covers when completely stretched out.
Similarly, a Ph.D. dissertation by Cynthia M. Hale (2004)$^4$ measured earthworms by preserving them and then
measur[ing] on the longest axis of the straightened individual.
It seems as though ranges of body length are almost always provided for each species. Although this is not uncommon for any taxa, many sources (including those cited) mention that variability in preservation methods/ body orientation can lead to variation in lengths recorded (as can age, vitality, gut content, moisture content, etc.). Klemm, in particular, suggests using numerous museum specimens to make conclusions about species because of these variability issues.
As a result of these discrepancies/inconsistencies, I would guess that you're best off citing a methodology source (e.g., Taube 1966) and explicitly indicating your measurement methods.
 Taube, Clarence M. 1966. Leeches. Institute for Fisheries Research Report No. 1713. http://quod.lib.umich.edu/f/fishery/AAG2862.1713.001?rgn=main;view=toc.
 Klemm, Donald J. 1982. Leeches (Annelida: Hirudinea) of North America. EPA-600/3-82-025. Cincinnati, OH.
 Klemm, Donald J. 1995. Identification guide to the freshwater leeches (Annelida: Hirudinea) of Florida and other southern states. Florida Department of Environmental Protection. Tallahassee, FL. http://publicfiles.dep.state.fl.us/dear/labs/biology/biokeys/leeches.pdf
 Hale, Cynthia M. 2004 Ecological consequences of exotic invaders: interactions involving European earthworms and native plant communities in hardwood forests. PhD dissertation. University of Minnesota.