I started using a 12GB C drive, but am having problems running out of space on VMs that have been around a while. MS hotfixes and other patches are eating up a lot os space. So recently started using a 20GB C drives, that should be good for a while.
I have found an easy way to resize the boot partition. Only takes a few minutes. Take a look at this link:
http://vmprofessional.com/index.php?content=2k3resize
