1. Shut down the VM if it is running;
2. On CLI, run:
VBoxManage modifyhd /directory/myWindowsVM.vdi –resize numberInMB
3. Start VM -> Manage Computer -> Disk Management
4. If you want to give the space to the existing drive, right click on said drive -> Extend Volume… and follow instructions
5. If you want to create a new drive, click on the new space, and do partition wizard, follow instructions.