when installing on virtualox we have to install Guest Additions, to be able to improve image resolution and better control of the mouse. :

- first way: from terminal 
pacman -S virtualbox-guest-utils => for VirtualBox Guest utilities with X support, then choose for the default linux kernel choose virtualbox-guest-modules-arch and for non-default kernels choose virtualbox-guest-dkms
pacman -S virtualbox-guest-utils-nox => for VirtualBox Guest utilities without X support
systemctl enable vboxservice
reboot

- second way: 
mount the VBoxGuestAdditions.iso on cdrom of virtualbox app
mount /dev/cdrom /mnt => this will mount the  cdrom
cd /mnt
./VBoxLinuxAdditions.run

========
NOTE:
========
we may need to have as graphical card VBoxVGA to be able to have full screen
so go to the virtualbox app => arch vdi setting => display => choose as graphics controller VBoxVGA