I've had dreadlocks for almost 2 months now, and yes, it will shorten your hair. My hair was to my waist before, and now it's about arm pit length. What method are you using, back combing? What ever you do, do NOT use wax! There are a lot of companies, such as Knottyboy and DreadheadHQ that push the use of wax, but all this does is glue your hair together and keeps it from forming knots, so it hinders the process in the long run. Plus its just nasty and greasy. Basically, wax is to dreadlocks as stretching too fast or skipping sizes is to your ears. It might show results at first but in the long run it is very bad and a hard process to reverse. If you need it to tame frizz, try pure aloe vera gel instead. Also, make sure you're in it for the long haul since they can take up to a year to mature and will look like shit for a while. It's best just to leave your hair alone, washing every so often with a natural, residue free soap. I use Dr. Bonner's and it works great. You can get that at any health food store or online.
http://com3.runboard.com/bknottydreadlockforum that is a great source for any dreadlocking information you might need. If you have any more questions feel free to ask
Here's a before an after pic of mine:
"If a composer could say what he had to say in words, he would not bother trying to say it in music." -Gustav Mahler