'Set or return the line thickness.
Aliases:  pensize | width
Argument:
width -- positive number
Set the line thickness to width or return it. If resizemode is set
to "auto" and turtleshape is a polygon, that polygon is drawn with
the same line thickness. If no argument is given, current pensize
is returned.
Example (for a Turtle instance named turtle):
>>> turtle.pensize()
1
>>> turtle.pensize(10)   # from here on lines of width 10 are drawn'

def pensize(self, width=None):

    if (width is None):
        return self._pensize
    self.pen(pensize=width)
