Printing ArUco markers at the right size
Every pose a detector reports is scaled by the marker size you gave it. If the printed marker is five percent small, every distance is five percent wrong, and nothing in the output says so.
Turn off fit to page
Print dialogues scale by default. The setting is called Fit to page, Shrink oversized pages, or Scale to fit, and it is usually on. It moves the marker by a few percent, which is enough to matter and small enough to miss.
Set the scale to 100 percent, or to Actual size. Print the PDF, not a screenshot, and do not let a viewer re-render it.
Check the sheet, do not trust the dialogue
Every sheet from this generator carries a ruler along the foot. Put a steel rule against it after printing. If the 50 mm mark is not at 50 mm, the sheet is scaled and the numbers on it are wrong.
The header of each sheet also records the settings that made it, so a printed board can be matched back to the numbers you gave the detector months later.
Paper and mounting
Use matte paper. Glossy paper throws specular highlights that wash out cells and cost you the marker at exactly the angle you wanted it.
A calibration board has to be flat. Paper curls, and a curled board quietly adds error to every frame. Mount the sheet on aluminium composite, glass, or foam board with spray adhesive, and keep it out of sunlight.
Never print a calibration board across several sheets and tape them together. The joint moves, and the board is no longer the rigid object the maths assumes.