Select an edge type from the list: Visible Edges

Thickness

Choose a thickness from the list, or select Custom Size and enter a Custom thickness value.

2010 SOLIDWORKS API Help - Document Properties > Line Font  http://help.solidworks.com/2010/english/api/swconst/dp_linefont.htm

Visible Edges - Custom thickness

IModelDocExtension::GetUserPreferenceDouble(swUserPreferenceDoubleValue_e.swLineFontVisibleEdgesThicknessCustom, swUserPreferenceOption_e.swDetailingNoOptionSpecified)

IModelDocExtension::SetUserPreferenceDouble(swUserPreferenceDoubleValue_e.swLineFontVisibleEdgesThicknessCustom, swUserPreferenceOption_e.swDetailingNoOptionSpecified, )