Returns or sets the amount of space between base lines in the specified text, in points or lines. Read/write Single.
Example
This example sets line spacing to 21 points for the text in shape two on slide two in the active presentation.
With Application.ActivePresentation.Slides(2).Shapes(2)
With .TextFrame.TextRange.ParagraphFormat
.LineRuleWithin = False
.SpaceWithin = 21
End With
End With