The Cueball, often abbreviated to cue, or even simply q, is the main unit of distance in the OTC. It is defined as the length of one (1) Cueball.
The exact conversion of a Cueball to Outside units is not known, but it is assumed that one cue is approximately 0.35 giraffes (1.75m; 5'9").
The Cueball is in many cases the only reliable unit of measurement in the OTC. Pixels aren't constant (because of zoom), and Outside units are not just useless, but also heretical.
According to this post, 1 cue = 0.37 giraffes = 1.87 m; although that was rebutted in this post.