Final answer:
To change the turtle's pen size to 6 pixels in Turtle Graphics, use the command pensize(6) or width(6), with the former being the standard method in Python's Turtle module.
Step-by-step explanation:
The command you would use to change the width of the turtle's pen size to 6 pixels in many programming environments that use the Turtle Graphics library is pensize(6) or width(6). While pensize is the method used in Python's Turtle module to set the thickness of the pen to a certain number of pixels, width is also used for the same purpose in some versions of Turtle graphics. Options 3 and 4, penwidth(6) and size(6), are not standard Turtle commands.