Limiting a standard user to use 50 GB of the hard drive
This should work for you:  


Create a root shell session in Terminal with sudo -s, providing your admin password when asked.
Create an empty quota options file with touch /.quota.ops.user
Run quotacheck -a to generate a list of drive space used by user
Run repquota -a to list the drive space used by each user
Enable quotas by typing quotaon / (You can turn them off again with quotaoff / )
Set quota limits for a given use by typing edquota -u username
This will put you in a vi editing session, where you can set both hard and soft limits on drive space and inodes (files).
Note: vi is a text editor and can be a bit of a handful to use if you're not used to it, try searching for a basic tutorial on the net if you've never used it before. 


You'll get a file that looks like this:  

Quotas for user fakeuser:
/: 1K blocks in use: 14968, limits (soft = 0, hard = 0)
        inodes in use: 224, limits (soft = 0, hard = 0)  


The soft limit can be exceeded for one file, allowing users to save a file which puts them over their limit but not allow them to save any more until they get back under their limit.  Hard limits cannot be exceeded though.