Cells.PreferredWidthType (Word)

Returns or sets the preferred unit of measurement to use for the width of the specified cells. 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.

With ActiveDocument.Tables(1) 
 .PreferredWidthType = wdPreferredWidthPercent 
 .PreferredWidth = 50 
End With