Column.PreferredWidthType (Word)

Returns or sets the preferred unit of measurement to use for the width of the specified table column. Possible return values are wdPreferredWidthAuto - Automatically select the unit of measure to use based on the current selection, wdPreferredWidthPercent - Measure the current item width using a specified percentage, wdPreferredWidthPoints - Measure the current item width using a specified number of points.

ActiveDocument.Range.Columns(1).PreferredWidthType = wdPreferredWidthAuto