The 'Line
width' command is used to set the active line width for the elements
drawn in ‘Draw’. After selecting the command, a dialog appears and you can
select a value between 1 and 16 or enter a larger value. This value is
limited to 100.
The numbers represent multiples of a width unit with one width unit equal to 0.01 inch, or 0.254mm.

Hint
You also have access to the current line width by using the information bar