Ctrl + seems to work fine to increase selection size by one pixel. But Ctrl - doesn't always work to reduce the selection by one pixel. It jumps all around. It even changes the shape of the selection. It reduces the selection, but not by just one pixel, and not by the same amount on each side every time.
I also wanted to suggest that when the program opens, the Rectangular selection tool be active by default instead of the Zoom tool. I use the keyboard + and - to zoom. Just a suggestion. Maybe you use the Zoom tool. I use the Rectangular selection a lot more.