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