Does anyone know of any scripts to use for setting up www.mydomain.com/username....

Basically I know I can set this up in Cpanel which is what I use currently but I want to be able to specify the amount of space/bandwidth that /username would recieve....

You can't find such a script...
It should not be "a script" if it will control quota/web server/traffic... bla bla

Then it becomes a complicated software...

well, I figured a way around it, I am just using username.mydomain.com for this now

There are many scripts out there, checkout hotscripts or cgi or php resource index.