This post shows How do I install VirtualBox Guest Additions on Debian 9 so you can resize your Debian 9 guest and also Copy and paste between your debian guest and Windows 10 / Windows 8 / Linux host.

Open the terminal. Follow the commands described below.

Login with root privileges

su -

the provide your root password.
Next run the following command in your terminal as root.

[debian9@localhost ~]$ su - 
Password: 
[root@localhost ~]# echo deb http://ftp.debian.org/debian stretch-backports main contrib > /etc/apt/sources.list.d/stretch-backports.list
[root@localhost ~]# apt update
[root@localhost ~]# apt install virtualbox-guest-dkms virtualbox-guest-x11 linux-headers-$(uname -r)

Now restart your Debian 9 Operating system.

That’s it! I hope this tutorial will be helpful. Thanks