Measure for Measure - Width and Height

From Draw Like da Vinci, p. 50: To gauge the proportions of an object, use the age-old trick of holding your pencil at arm's length (the arm must be outstretched and kept still). Close one eye; make sure you always use the same eye! For assessing height, hold your pencil point up so its tip aligns with the top of the object. Slide your thumb along the pencil until the thumb aligns with the lower end of the object. Without moving your thumb, rotate the pencil to a horizontal position while still keeping the arm outstretched. Now you can determine the relative height and width of the object.  

Having assessed the overall height against the width, you can gauge more proportions and mark them on your page. Fitting your subject in a box that defines the height and width of your drawing can also be helpful. Leonardo da Vinci was fond of using squares and rectangles to assess proportion.