This is my "First Layer Target". A 200 x 200 x 0.20 crosshair shaped target object which I use to test how good I levelled my print bet either manually or automatically. At the best setup it should print with exactly one layer at 100% layer height and have an even look everywhere on the bed!
I also included a 200 x 200 x 5 version which can be used to measure your x / y distortion across the whole printing area. This is much better than trying to do this on a 20x20x20 cube. Still I don't believe much into adjust steps as those are basically a constant for a given design. But that is another topic.
You can of course resize it to the size of your print bed! The 5 mm thick version is nothing else than the original with a different height and I made that just for convenience.