This is a calibration cube designed in OpenSCAD. The letters should be 12mm tall and 11mm wide if when measured in their correct orientation. The cube itself as the title suggests should be 20mm in each dimension.
Do remember that if you're printing with a first layer height less than 100% your measurement in z should be - layer height - (layer height * (first layer % /100)) shorter than 20mm.
Also note that if you print this in layer heights which aren't exact divisions of 1mm then the letter heights may not be exact!
The letters should appear centrally on the face they are printed on (both horizontally and vertically!)