Ran into the same issue when I installed PSE 2018 and now I can't remember how to fix this! I want the gray "frame" around the images in the work space to auto-size with the image size, if that makes sense to you. I don't know how else to describe it. In other words, when I change the image size on-screen using Ctrl and - or Ctrl and +, I don't want to see a gray, fixed size background to the image.
I'd greatly appreciate if someone can refresh my mind on this.