Search for "XYZ 20mm Calibration Cube." It is the gold standard for dimensional accuracy.
While most users download STL files and slice them personally, downloading pre-sliced G-code can be helpful for testing specific hardware configurations. download gcode cube
Once your download is finished and the cube is printed, grab your digital calipers. You are looking for three specific things: Search for "XYZ 20mm Calibration Cube