"cm-ruler" is a 10 cm long ruler with 1 mm ruling and a hole each cm, for viewing in a CAD tool or printing. The "5mm-ruler" version has one line every 5 mm, and larger size ruling lines. Outside nominal dimensions of both are 100 x 10 x 2 mm.
This is a remix of http://www.thingiverse.com/thing:3077 in which I reorganized (and I think, corrected) the OpenSCAD source code.
I recommend printing slowly for better quality. With Ultimaker 2, my 20 mm/s test print definitely looked cleaner than the 40 mm/s although both had similar accuracy for outside dimensions.
My results agree with a value of 0.2% for the PLA shrinkage after printing. In other words, I would need to scale up this model by 1.002 for the resulting print to measure accurately.