sudo VBoxManage modifyhd "vdi location" --resize "new size in MB"
Please note that if your location address contains spaces it's better you insert it within single quotes as I've done in the above example to avoid error in the terminal.
sudo VBoxManage modifyhd '/home/evagabond/VirtualBox VMs/Windows/Windows 7 (x64)/Windows 7 (x64).vdi' --resize "61440"
Finally if successful you should get result as below:
Now you will need to resize/expand your VHD partition using a Partition Software or Windows Default Partition Manager. I have used a free software called MiniTool Partition Wizard which you can download here: http://www.partitionwizard.com/download.html
Follow the instructions in the image below to finally expand your partition: