The marker size should be exactly 14cm for slam purposes, but it shouldn’t matter too much for fiducial follow. If you want a specific marker size you will need to play with your printer settings to make sure that you are printing 1:1 scale.
Camera calibration is not required for fiducial follow, although it will make it marginally more accurate. It is highly recommended for fiducial slam @johnnyv.
@inmoov try moving further back from the magni and holding the marker higher up so that it is in the field of view. I would try ~4 feet or ~125cm off the ground.