Ah ... I think I understand your setup. You are doing things in a strange way. Servers don't have a GUI. They text resolution is controlled by grub and normally 80x25 or 132x50 characters are used. There is no copy/paste in doing this. Might want to check out the "scale" controls for the VM window. Also, since there isn't any text copy/paste, would be good to practice up on file and command redirection into files.
Servers should be managed 99.99999999% of the time using ssh. From Windows, most people use putty as an ssh client. It allows copy/paste in a reasonable way between the windows and different guests. Normally, only a complete noob would install X/Windows onto a Linux server. It just isn't done for a number of reasons. There are exceptions for the GUI on a server, but not many. If you want a GUI on Linux, install a desktop like LUbuntu/Xubuntu or Ubuntu-Mate. Avoid the heavy desktops like Gnome3 or KDE inside a VM. Virtualized systems don't work with with heavy graphics.
Also, the guest additions usually aren't used by Linux server installs. They provide mostly end-user/GUI features which just aren't needed/wanted. The guest additions have some security problems too.
Another item - the paravirt interface. You probably want to use virtio, if that is an option. Disabling it will have performance impacts on the host and the guest. I don't use virtualbox. Below is what KVM (a different hypervisor used by enterprises) shows with virtio NIC and HBA:
Code:
$ lshw -c network
*-network
description: Ethernet interface
product: Virtio network device
vendor: Red Hat, Inc.
and for the disk controller ...
Code:
*-scsi
description: SCSI storage controller
product: Virtio block device
vendor: Red Hat, Inc.
So, on the ubuntu server, sudo apt install openssh-server fail2ban.
Ensure the vbox networking is setup for "Bridged", not host-only or NAT. If you had to change the networking in vbox, reboot the ubuntu server. Otherwise, continue ...
On Windows, open putty and connect to the ubuntu server IP on the LAN using your ubuntu userid. Now you can select text with the left mouse button and paste it with the right. You can resize the window, change the fonts all inside putty. This doesn't add the bloat of a GUI, it doesn't drastically slow down your VM or your host and it doesn't introduce all the security problems that come with running a GUI on a Unix system.
If you want a more convenient and more secure system, setup ssh-keys for authentication between Windows and Linux. I bet there are at least 100 guides for that. Using passwords is so, so, so, 1995. Passwords are bad for security. If you can easily type a password, it is bad.
I'm 95:5 that this is really what you need, but only 50:50 that this is what you want. I run lots of virtual machines around the world. 99.999999% of the time, access for management is via ssh. About once a year across all the systems, I'll need direct console access for 1 box. ssh is how system administration is performed for Unix.
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