Single layer test square 100x100

Square 100x100x0.1mm with 2mm sides is for a simple bed leveling test.
Some pros and cons:
+: minimal printing time and plastic consumption
+: you can check leveling of all 4 bed corners
-: you can't be shure that middle point of bed is ok

Please note that layer height can be too low for your slicer software. Just scale Z size to 200% in that case.