Make a note of your Operation System current resolution. Let’s say 1900×1080
Go to the VirtualBox installation folder open command prompt in the folder. Usually in C:Program FilesOracleVirtualBox or Open up a command Prompt and navigate to folder where VBoxManage.exe is..
Type this command into the command prompt: VBoxManage.exe setextradata “VM-Name” CustomVideoMode1 1920x1080x32
Open up the Windows 10 and from resolution center change the resolution to your new resolution.