"cm-ruler" is a 10 cm long ruler with 1 mm ruling and a hole each cm, for viewing in a CAD tool.  Outside nominal dimensions are 100 x 10 x 2 mm. Slow print speeds work best. If you want to print for best accuracy, you would scale up the model by about 1.002 (in PLA) or 1.005 (in ABS) to balance the plastic shrinkage. The exact scaling factor depends on your filament, printer settings etc. This is a remix of http://www.thingiverse.com/thing:3077 in which I reorganized (and I think, corrected) the OpenSCAD source code.