Adjust the image window to match your monitor resolution
The first time I used GIMP was on a monitor which could only display 256 colors. If you have never had the pleasure of using a monitor like this to make images with, you have missed quite a bit. For example, if I used text highlighting in the text editor, the computer would steal colors from the rest of the display to use for this. It was really impressive to see it work, actually. From this experience, I learned to trust the numbers of the colors and not what my eyes were seeing.
On such a monitor, square images were never square. Even when I started to use newer displays with more display area and more colors monitors, square images usually drew on the screen taller than they were wide. GIMP image display windows can be adjusted to draw themselves in a way to make up for the difference.
better GIMP how tos
The information I write here is often laced with self doubt, thinly disguised insults and an occasional flirt with people I have either met or imagined.
Better documentation about all of this can be found at docs.gimp.org.