Thanks for your help, I thought it should be as simple as changing the size
but I'm still struggling.  I have tried using the two lines you suggested but
got no further.  

The code I am using is:

Sub AddTextBoxDateFilename()
' Adds a text box with date and filename to each slide
' You must first save the presentation at least once before using this

   Dim oSl As Slide
   Dim oSh As Shape

   On Error GoTo ErrorHandler

   For Each oSl In ActivePresentation.Slides
       ' do we already have a filename/date text box?  If do, use it:
       On Error Resume Next
       Set oSh = oSl.Shapes("FilenameAndDate")
       On Error GoTo ErrorHandler

       If oSh Is Nothing Then  ' no text box there already, create one

           ' change the position and formatting to suit your needs:
           Set oSh = oSl.Shapes.AddTextbox(msoTextOrientationHorizontal, 0,
510, 720, 28.875)

           With oSh

               .Name = "FilenameAndDate"

               .TextFrame.WordWrap = msoFalse
               With .TextFrame.TextRange.ParagraphFormat
                   .LineRuleWithin = msoTrue
                   .SpaceWithin = 1
                   .LineRuleBefore = msoTrue
                   .SpaceBefore = 0.5
                   .LineRuleAfter = msoTrue
                   .SpaceAfter = 0
               End With

               With .TextFrame.TextRange.Font
                   .NameAscii = "Arial"
                   .Size = 8
                   .Bold = msoFalse
                   .Italic = msoFalse
                   .Underline = msoFalse
                   .Shadow = msoFalse
                   .Emboss = msoFalse
                   .BaselineOffset = 0
                   .AutoRotateNumbers = msoFalse
                   .Color.SchemeColor = ppForeground
               End With
           End With    ' shape

       End If  ' osh is nothing

       ' now we know there's a shape by the correct name so
       Set oSh = oSl.Shapes("FilenameAndDate")
       With oSh.TextFrame.TextRange
           .Text = ActivePresentation.FullName & vbTab & Format(Now, "mmmm
dd, yyyy")
       End With

       Set oSh = Nothing

   Next    ' slide

NormalExit:
   Exit Sub

ErrorHandler:
   MsgBox ("There was a problem:" _
       & vbCrLf _
       & Err.Description)
   Resume NormalExit

End Sub