Make sure you have the Oracle VM Guest Additions installed. Once I installed the latest according to the version of the VirtualBox I installed on my host OS everything worked like a charm.
On your Windows 10 Virtual Machine, at the lower right-corner of the screen you'll see a
^. Click on it, and you'll see the VirtualBox Icon Running. If you see that but you still can get full resolution changes are you have the incorrect version installed.
Windows File Explorer, click on the CD Drive containing the
Virtual Machine Addons and install it.
Alternatively you can also hover your mouse towards the top of the screen to expose the VirtualMachine Menu, Select
Devices and select
Insert Guest Additions CD Image.