Open a Command Prompt Window (Run as Administrator)
Change to VirtualBox’s Program Files Folder
Then type the code below in the Command Prompt.
VBoxManage modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MB
replacing YOUR_HARD_DISK and SIZE_IN_MB with your Hard Disk name and desired size.
Like,
VBoxManage modifyhd D:\VB\Win10\win10.0.vdi --resize 20480