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 ./

Then restart again, and select “Auto-resize Guest Display”, click full screen and exit full screen, then you can resize the debian window.