I would have added this to my Friday aggregate post, but I didn’t see it until just now.
The ever-helpful Fat Bloke has a post on Growing your VirtualBox Virtual Disk that I highly recommend you Pocket, bookmark or whatever you use to keep useful links handy for later use.
I actually went through this very process myself to upgrade my Windows 8 Consumer Preview VM to the released version, which needed additional space to perform the upgrade.
The process is pretty straightforward, use the command:
VBoxManage modifyhd <uuid>|<filename> [--resize <megabytes>|--resizebyte <bytes>]
Then boot the VM and don’t forget to expand the partition to include the newly allocated space, like so in Windows 8:
Anyway, save this for later because I guarantee you’ll need it at some point.
Update: While Fat Bloke mentions installing GParted from Ubuntu’s LiveCD to resize a partition, Bill Taroli (@btaroli) suggests, based on his experience, going straight to the source and using the GParted LiveCD as another course of action.
The more you know.