This happened to me too. I had to grab the box that comes up and resize it like I would with a normal window, mine glitched a lot when I tried it, try resizing it as far as you can, it will try and glitch back, but just keep fighting it until it becomes a usable size, then log out of Plasma and log back in, and then you can size it back down to a normal size. Hopefully there will be an official fix for this soon
Ive only had to setup a nvidia system once, so I might be missing some packages, but I think
pacman -Rns nvidia nvidia-utils lib32-nvidia-utils
should get rid of all of it.