This article describes a custom creation, custom theme, or other fan material, made by a Brickipedia contributor. It has never been, is not, and will not be officially released.
I've seen some of these floating around lately, so I decided to try it myself!
XS stands for 'Extra Small'.
The whole motor would be on a 2 x 2 base, and be about 5 plates high.
(Pictures soon, gotta draw 'em!)