What is the minimum length of hair you can donate to be made into wigs for cancer patients?

I've been growing my hair out for a few years now because I'd like to donate it so it can be made into wigs for cancer patients, or other people who have lost their hair. My hair is long, curly, and extremely thick, and it seems to grow ridiculously fast. I don't want to shave it all off, though -- is it acceptable to just get it cut off to your shoulders and donate the rest, or does it have to be shaven for some reason? I was thinking I'd let it grow out to my waist or the small of my back before I had it cut. (It's already midway down my back) Would that be okay?