Sight size is a wonderful method of drawing and painting in which the drawing and the subject are the exact same size. From the flat this is quite easy and much more obvious. If you are working from life or 3 dimensions, they appear the same size from a stationary point at a distance.
Working from life sight size we have the challenges of making sure we come back to the same spot every time (even seeing your set up from a slightly different spot makes a big difference), and that our measurements are not super accurate.
However, by starting out working from the flat, we have the opportunity to get very accurate very quickly. We can also use this opportunity to practice training our observation.
For example, when I work on portraits, I will combine a photoshopped image reference with a color study or work from life. I print out my photoshopped reference at size and work on the drawing sight size because I can get to an accurate drawing faster.