Quotas allow you to configure disk usage limits on a per user/per volume basis. You can not set a quota over multiple volumes.
Quotas are on file size and even if files are compressed, they count against your defined quota based on the uncompressed size of of the files. For example, If you had a 50MB quota and compressed your 50MB of files using NTFS compression, you still would have used all your quota.
Quota support is only available on NTFS 5.0 volumes and is enabled as follows:
Start Explorer
Right click on the volume you wish to enable quotas on and select properties >> Quota tab >>
Check the ‘Enable quota management‘ box
You can set default options for new users and also set the actions to take if quota exceeded, either deny disk space or allow them to carry on. There are also various logging options, either when a user exceeds their warning level or when they exceed their actual quota.
Click Apply
A warning will be given. Click OK.
The quota process will now check the volume and build up a list of current disk usage.