Sight-Size is a method of drawing an object exactly as it appears to the artist. It is done on a one-to-one scale. It was developed in the 19th century. The rules are rigid and there is like room for creativity however, it builds your ability to see, replicate and render objects from direct observation. It is a technique that is used widely in many workshops/ateliers in Florence.
Step by step instruction by Tim McGuire
Sources: http://www.explore-drawing-and-painting.com/drawing-tips.html