Have a look at the bottom of this screenshot from vlite.net, if all components are selected you can slice a massive 4666.9MB of stuff from an install
With everything selected. No matter you choose maximum or fast compression, you will get a 1GB iso (assuming you selected Ultimate Edition).
Once installed Vista will still take 4GB.
(speaking of x86 version of course)
PS: maximum compression is really not worth the time compared to fast. Don't use it. Here it saves 40 to 60MB and needs an extra 20 minutes.