__set-line-thickness
__set-line-thickness number

Specifies the thickness of lines and outlined elements in the turtle’s shape.
The default value is 0. This always produces lines one pixel thick.
Non-zero values are interpreted as thickness in patches. A thickness of 1, for example, produces lines which appear one patch thick. (It’s common to use a smaller value such as 0.5 or 0.2.)
Lines are always at least one pixel thick.
This command is experimental and may change in later releases.