Using sheets 7 1/2" high (layers of brick running left and right) is how as many as a dozen chimneys can be made at one time. The square styrene tube shown here is 14" long and the sheet material only covers
a little more than half.  I suggest leaving the tube long and using that as a "handle" during the project.  When the chimneys are used, the remainder of the tube can be used for another section (stick in a pencil if you are comfortable working with a handle).  The tube used in the clinic is Evergreen 5/16" (.312x.312) styrene tube that comes two in a package with pieces 14inches long. You may use other sizes and rectangular tubes as well.