Sight size means to draw or paint your model in the size you see it, from a given distance. The artist's goal is to compare what he sees and capture it as well as possible on the canvas by a constant comparison of shapes and angles from afar.
For this to work best the easel has to be placed physically as close to the model as possible. Then by stepping back the artist should be able to simultaneously see both his work and the model, without having to adjust his position or try to focus on one or the other.
Note that the artist has to step back from the easel to look and compare placement and then walk forward to place his mark on the canvas. Leonardo Da Vinci suggest: "When drawing from nature, stand back three times as far away as the object you are drawing". This means that the artist has to take the larger measurement of either the width or length to determine how far back he needs to stand. Once this measurement is found, place a piece of tape on the ground so you can easily find your viewing position. The average still life set up would require the artist to stand about 6 to 8 feet away from the set up.