Kim,
This does it for the first shape on Slide 1. You'd also probably want
to position it using a calculation using .top and .left as well.
With ActivePresentation.Slides(1).Shapes(1)
.Height = 0.9 * ActivePresentation.Slides.Range.Master.Height
.Width = 0.9 * ActivePresentation.Slides.Range.Master.Width
End With