Window.UsableWidth (Word)

Returns the width (in points) of the active working area in the specified document window.

If none of the working area is visible in the document window, UsableWidth returns 1. To determine the actual available height, subtract 1 from the UsableWidth value.

With ActiveDocument.ActiveWindow 
 MsgBox "Working area height = " _ 
 & .UsableHeight & vbLf _ 
 & "Working area width = " _ 
 & .UsableWidth 
End With