What are the chances that a virus, malware, spyware or trojan could propogate from a Windows Virtual Machine to your host Mac OS X?
In other words, if I delete the virtual machine that contained the nasty whatever, is that all I would need to do?
Theoretically, it's possible. In practice, the answer depends on exactly what you mean and your setup, but OS X probably isn't in danger.
If you don't have Shared Folders or any other guest-host communication set up, I'm not aware of anything which would allow the guest to access the host (this is precisely one of the things that virtualization is supposed to prevent), the risk is pretty much zero.
If you do have Shared Folders, and are asking if OS X could be affected, malware would have to be able to run on both the guest and the host. OS X has been pretty untargeted, much less via multiplatform malware, so that's not much of a worry.
I think the biggest concern would be if you had a Shared Folder (or similar) set up to multiple guests - then the Shared Folder could act as a carrier between them, just as your Mac can act as a carrier for malware (e.g. that suspicious exe file in your email probably won't affect Mail.app, but could be bad if you forward it to someone else).
(1) I do have shared folders enabled at power on
One easy thing to do is to not give write access to the guest unless you need it, and to share only as much as you need.
(2) The master hard drive where Fusion is installed reduces its size by the number of GB of RAM that I use for the VM when I load the VM -- for a data or swap file, I am assuming.
Yep - a powered on virtual machine has a .vmem file in the .vmwarevm bundle (by default, though there are a couple cases where it's located elsewhere) as big as the RAM allocation.
