I am able to grow the rectangle horizontal and vertical via following code.

But I want to grow the rectangle only horizontal.

Sub Macro1()

'Delete all slides
For i = ActivePresentation.Slides.Count To 1 Step -1
    ActivePresentation.Slides(i).Delete
Next i

'Add a blank slide
ActivePresentation.Slides.Add 1, ppLayoutBlank

'Add a Rectangle
With ActivePresentation.Slides(1).Shapes.AddShape(Type:=msoShapeRectangle, Left:=100, Top:=100, Width:=200, Height:=50)
    .Name = "myRectangle"
End With

'Add a GrowShrink effect to the myRectangle
With ActivePresentation.Slides(1).TimeLine.MainSequence.AddEffect(Shape:=ActivePresentation.Slides(1).Shapes("myRectangle"), EffectId:=msoAnimEffectGrowShrink)
    .EffectParameters.Size = 150
    'The following code doesnt work
    '.EffectParameters.Direction = msoAnimDirectionHorizontal
End With

End Sub

Thanks in advance.