Dim swApp As ObjectDim Part As Object
Dim boolstatus As Boolean
Dim longstatus As Long, longwarnings As Long

Sub main()

   Set swApp = Application.SldWorks

   Set Part = swApp.ActiveDoc

   Dim myTextFormat As Object
   Set myTextFormat = Part.Extension.GetUserPreferenceTextFormat(swUserPreferenceTextFormat_e.swDetailingDimensionTextFormat, swUserPreferenceOption_e.swDetailingDimension)
   myTextFormat.CharHeightInPts = 20
   boolstatus = Part.Extension.SetUserPreferenceTextFormat(swUserPreferenceTextFormat_e.swDetailingDimensionTextFormat, swUserPreferenceOption_e.swDetailingDimension, myTextFormat)

End Sub

The above code successfully changed the font size of the dimension lines in the document properties of the drawing, but the font size of existing dimensions on the drawing was not changed accordingly.