Change Resolution to MacOS Guest
cd "C:\Program Files\Oracle\Virtualbox"
VBoxManage setextradata "MacOS Sierra" VBoxInternal2/EfiGopMode 3
Where N can be one of 0,1,2,3,4,5 referring to the 640x480, 800x600, 1024x768, 1280x1024, 1440x900, 1900x1200
0 1 2 3 4 5