Document.ReadingModeLayoutFrozen (Word)

Sets or returns a Boolean that represents whether pages displayed in reading layout view are frozen to a specified size for inserting handwritten markup into a document.

Use the ReadingLayoutSizeX and ReadingLayoutSizeY properties to specify the size of the pages displayed when the reading layout size is frozen for inserting handwritten markup into a document.

ActiveWindow.View.ReadingLayout = True 
ActiveDocument.ReadingLayoutSize 300, 300 
ActiveDocument.ReadingModeLayoutFrozen = True