The time will come when there’s a need to increase the Virtual disk size. These are the steps (to increase existing size of 10GB to 20GB):
STEPS
1. Close the VM VirtualBox Manager.
2. On command prompt, go to the VirtualBox directory & issue command VBoxManage modifyhd “d:\vm\xp.vdi” --resize 20480
Notes :
d:\vm\xp.vdi is the location of the vdi file. The location can also be found under SETTINGS – STORAGE.
20480 is the size in MB. In the case above, 20GB x 1024MB = 20480MB
The progress will be shown. Resize is completed at 100% 
3. Start VirtualBox Manager. The VDI size is now at 20GB. 
4. Boot up the virtual machine.
5. Final step is to Increase the partition size. My favorite is EaseUS Partition Master Free Edition. It’s simple & easy to us. Best of all, it’s free.
