Frameset.WidthType (Word)

Returns or sets the width type for the specified Frameset object. Possible return values are wdFramesetSizeTypeFixed - Microsoft Word interprets the height or width of the specified frame as a fixed value (in points), wdFramesetSizeTypePercent - Word interprets the height or width of the specified frame as a percentage of the screen height or width, wdFramesetSizeTypeRelative - Word interprets the height or width of the specified frame as relative to the height or width of other frames on the frames page.

With ActiveDocument.ActiveWindow.Panes(1).Frameset 
 .WidthType = wdFramesetSizeTypePercent 
 .Width = 25 
End With