If you are already root (unlikely, but possible) you can remove the
sudo -s part. Here's the command:
sudo -s du -sm /Users/* | sort -nr | head -n 10
sudo -s or it will give a few Permission denied errors before finally spitting out the results, and they may be incorrect. The directory structure should start with the /Users Directory and then it will recursively perform the operation. This may miss any folders outside of the normal user space, but there shouldn't normally be any user data there.
[crarko adds: I tested this, and it works as described. There are third party utilities that will give the same results with a GUI, but this is pretty handy to have for use on remote machines over an ssh connection. It can take a while to complete, so be patient. Also I noticed in Activity Monitor the
du process was using a lot of cpu, so it's best to do this while not running other cpu (or I suppose disk) intensive programs.]

