ColumnWidths is an option for GridBox which specifies the widths of columns in ems.
The default setting is ColumnWidths->Automatic, specifying that all columns should be made wide enough to fit their contents without breaking onto multiple lines.
ColumnWidths->n uses column widths equal to n times the current font size—usually about n times the width of an M in the current font.
ColumnWidths->, , ... can be used to specify different widths for different columns. If there are more columns than entries in this list, then the last element of the list is used repeatedly for the remaining columns.
An explicit setting for ColumnWidths overrides ColumnsEqual->True.