My OS: Debian 8.4.0
Follow the guide above, below is the solution:
Click “Insert Guest Additions CD Image..”, then in console type:
apt-get install dkms apt-get update apt-get upgrade
Then restart, and type in console:
mkdir /mnt/cdrom mount /dev/cdrom /mnt/cdrom cd /mnt/cdrom sh ./VBoxLinuxAdditions.run
Then restart again, and select “Auto-resize Guest Display”, click full screen and exit full screen, then you can resize the debian window.