First, open a command line window and do this:
C:\Program Files\Oracle\VirtualBox>vboxmanage modifyhd "[your vdi path]" --resize 61443
where [your vdi path] is the path to your .vdi file.
And the number above, 61443, is in MB, so that’s a resize command to make the new drive 60GB.
Next, the partition of the existing drive needs to be increased: Open “Disk Management” (this is Windows), right-click on the current partition and select “extend” and follow the wizard.
To find the disk management admin utility, type “disk management” in the Windows “finder”.