UsableWidth Property
Returns the maximum width of the space that a window can occupy in the application window area, in points. Read-only Double.
Example
This example expands the active window to the maximum size available (assuming that the window isn't already maximized).
With ActiveWindow
.WindowState = xlNormal
.Top = 1
.Left = 1
.Height = Application.UsableHeight
.Width = Application.UsableWidth
End With