Steps to Increase your VirtualBox Hard Disk Size
cd into the directory where your virtual drive is found (usually ~/VirtualBox VMs/{Your VirtualBox PC name})
cd ~/VirtualBox VMs/Win7
run the modify hd function with the new size (in MB) I choose 50GB (50 * 1024mb = 51200MB)
VBoxManage modifyhd Win7.vdi –resize 51200
VirtualBox will resize your hard drive and you will see the following output from the system indicating that the process is complete
0%…10%…20%…30%…40%…50%…60%…70%…80%…90%…100%
Now start up your Virtual Windows Machine.
Start your partition manager and extend the partition.
Once complete you have successfully Increased your VirtualBox Disk Size, and can now continue installing applications and software on your VirtualBox OS.