Why would a block of wood, a small makom patur, in the middle of a reshut divide the reshut? And how would one know in which direction to draw the dividing line, since two points are needed to determine a line?
Joel P. Wiesen, Newton, Massachusetts
The dimensions given in the Rashbam are only for the Pisla's height and width. It's length is as long as the lenght of the Reshus (i.e. it separates the two sides entirely); see Rashi Shabbos 80a.