Hi Claus,

welcome to the MoIers!
Moi is a endless thing to play (and work) with....but (as far as i know) the line width in Moi is ZERO... its not like in a normal drawing where you use different line widths.
I know one script from MaxSmirnov "FatLines" which takes a line and draws a offset around. if you use the planar feature of Moi you can convert it to a face.
Pherhaps this is what you looking for.
A good way to start with MoI is here: http://moi3d.com/resources where you find the "MoI help file online" and also the link to the script resources.

But after about 4 years working with MoI i still find new things, tricks and features... endless ;-)

Have fun with MoI
Ralf