Hi Catty ,
Well i surfed the net for the solution to your problem , then tried the following code and it worked. I hope it works for you as well.

Sub LineSpacing()
With Selection.ParagraphFormat
.LineSpacingRule = wdLineSpaceExactly
.LineSpacing = 1.3 * Selection.Characters(1).Font.Size
End With
End Sub

Please revert back if it works.
Thanks
~Dex