To resize a disk in Virtual Box, you need to use the command line prompt and a VB tool called VBoxManage. To do this:
1] Open elevated command prompt
2] Change Directory to the Virtual Box installation folder, most likely C:\Program Files\Oracle\VirtualBox
3] Enter the following code:
VBoxManage modifyhd (insert path to visrtual disk) --resize (enter size in MB you wish to increase the disk to)
You should end up with something which looks like this:
This will have increased the size of the disk but you'll need to log into Windows on the VM and extend the Volume size in Disk Manager to be able to access the new space:
Follow the wizard steps through and you'll be left with a new, improved volume:
