Set the pen's thickness.
size input must be either a positive integer that denotes the pen's width or a list of the form [width height].
size input is a list, SETPENSIZE ignores the height component, but this may change in a future release.
To ensure that your programs are compatible with future versions of FMSLogo, you should either use a height that is the same as the width, or not use the list input at all.
SETPENSIZE 5 REPEAT 4 [FORWARD 100 RIGHT 90]