No matter what size hard disk you have, sooner or later you’ll find it’s too small. This is just as true for virtual hard disks as it is for physical ones. One of the benefits of virtialization is that you can change your ‘hardware’ setup on a whim. Want an extra gigabyte of memory? No problem, just assign it in your virtual machine config. So long as your host machine has capacity and the guest O/S supports it, this is easy to do.
Frustratingly though, none of the main VM software vendors (VMWare, Microsoft Virtual PC / Server and Sun VirtualBox) offer a way to resize a disk from the standard user interface. It is possible, but it takes a little fiddling to get it to work.