Hi ggagnon, there isn't currently any way to set it on an individual basis (I do want to add that in the future), but there is a way to change the default for all lines.

To do that, you will need to edit the moi.ini file with a text editor like notepad. You can find moi.ini here:
C:\Documents and Settings\[your login name]\Application Data\Moi\moi.ini
(or on Vista it is under C:\Users\[your login name]\AppData\Roaming\Moi\moi.ini)

Close down MoI first before editing the moi.ini file (since settings will be written to the moi.ini when you exit the program, overwriting any direct edits).

Inside of the moi.ini file, there is a [View] section, and inside of that there is a setting:

LineWidth=1

If you edit that to say something like LineWidth=3, that will make MoI use thicker lines for its display of curves and edges.

- Michael