Q: How do I increase the size of the virtual disk in Virtualbox?

A: The same command is used on both Windows and Linux. Below is an example for Windows, using a CLI-command.
Note that the command(s) are all typed on one row!
c:\ >"C:\Program Files\Oracle\VirtualBox\VBoxManage" modifyhd "d:\VMs\CentOS v5 x64 Clone.vdi" --resize 40960
# cd /home.2TB/VMs
# vboxmanage modifyhd "Win7 Pro SP1 x64.vdi" --resize 40960