Document.ReadingLayoutSizeX (Word)

Sets or returns a Long that represents the width of pages in a document when it is displayed in reading layout view and is frozen for entering handwritten markup.

After setting the ReadingLayoutSizeX and ReadingLayoutSizeY properties, use the ReadingModeLayoutFrozen property to display the page using the specified height and width. Use the ReadingLayout property to display a document in reading layout view.

ActiveWindow.View.ReadingLayout = True 
ActiveDocument.ReadingLayoutSizeX = 300 
ActiveDocument.ReadingLayoutSizeY = 300 
ActiveDocument.ReadingModeLayoutFrozen = True