The complicated steps - which you may learn other useful stuffs after going through them:
https://forums.virtualbox.org/viewtopic.php?p=29276#29276
The easiest and simplest method, which you can gain the result in just a minute or less:
1. Open a terminal window (well...here I am using a Macbook Pro).
2. Go to your VM directory (Mine is /Users/lpbinh/VirtualBox VMs/WIndows_7)
3. Issue the command "VBoxManage modifyhd <your VDI name> --resize <size_in_MB>
4. Since I am resizing a Windows VM, I have to boot up the Windows VM and perform the following additional steps:
a. Right click on My Computer, choose Manage.
b. Click on Storage, then Disk Management
c. Right click on the current partition, then select Extend Volumn. Follow the wizard to complete.
Well...take more than a minute, probably two :).
