Increase VirtualBox Hard Disk Size
From the Host Command line use the VboxManage resize option to increase the disk capacity
VBoxManage modifyhd \path-to-vdi-location\Windows\ 10\ Pro.vdi --resize 35840
This example is from a Linux host and increases the total size of the disk to 35GB (35 * 1024MB)
Once this is completed start the image and and use Windows Disk Manager to extend the volume to use the unused partition space.