Installing Arch Linux is ONLY THE BEGINNING... Then you get the joys of maintaining it... Arch is more than an OS... it is a LEARNING EXPERIENCE. I would suggest keeping an Arch machine to use as a daily driver... KEEPING regular backups... this way you can roll back changes if there is breakage... Always READ the wiki and the latest news releases before updating... those pages contain critical information in the even user intervention is required for an upgrade...Once you get used to using an Arch based system, the idea of using anything else becomes unthinkable UNLESS you have a server which requires stability... I use Debian for that...
Follow the tutorial video... take notes... READ THE WIKI... try again... Persistence is KEY!
sudo pacman -S virtualbox-guest-dkms
You install guest additions with pacman! NOT the guest additions ISO!
log out then back in... your virtualbox desktop will scale to your native screen resolution.