Ravi,
Here's a link to the requirements post on our blog - the requirements are in a file attached to the blog post.
http://blogs.technet.com/proclarity/archive/2008/08/20/proclarity-analytics-platform-6-software-and-hardware-requirements.aspx
I don't see specific disk space requirements for PAS 6.3. However, my educated guess is about 1GB minimum for the install and the intial web cache we build. Most user data is stored on the SQL server - but, that file is generally pretty small too - depending on your PAS deployment. Users are able to upload files to the PAS web server and you will want to take their total size into account as well.
Space requirements come down to your needs. But, several GBs would be a good recommended starting point.
-Joey
