The trick ozitraveller is to choose the "dynamically allocated" Virtual Hard drive. I always choose 30Gb's as my size. However since it is dynamic it only uses the space that your install requires. This way your not using all your drive space, as would happen in a "Fixed Size" Virtual Hard drive. The dynamically allocated hard drive will only use space on you actual physical hard drive until it fills up to the actual 'fixed size' you select in the setup of the VM, thus saving tons of space.
Hope this helps...
I think at the moment I have 13 functioning VM's using about 30Gb's of space total. That's with each one of those 13 VM's having 30Gb's assigned to them. If I was using the fixed space option that would be taking up a total of at least 390GB! Like I said, "Dynamically Allocated" is the way to go...