One of the advantages with virtual machines is that you can be very flexible with their sizes. If the host system permits it, you can have a very large virtual machine with a lot of virtual RAM and disk. Also, you can decide to go the other way around, to give the virtual machine a very modest amount of RAM and disk and then choose and configure the OS appropriately.
The question is, how small virtual machines have people managed to setup (and get to both boot up and to run)? Virtual machines doing something usuful is preferable, even if I know "useful" in this context is awfully subjective, but laboratory-cases with a configuration stripped beyond common sense could be intresting as well, just to see what people manage to boot and run.
Quite open ended question and quite academic, but think of it: an extremely small VM (which still does something useful) takes very little memory and disk and can be quite quickly saved to and restored from disk. If it's also gentle on CPU resources, one might consider having a huge number of such VMs up and running on a host.
(Imagine a VM running just an old Commodore 64 or Commodore Amiga in it. Ok, way wrong CPU architecture for modern Virtualization software running on a x86-based PC but still an interesting thought. You could have quite a few such small VMs running on a modern PC.)