Experiment with the following:

'----------------------------------------------------------------------
Public Sub TestCommentSize()
Dim rngA1 As Range
Dim comA1 As Comment

Set rngA1 = ActiveSheet.Range("A1")

With rngA1
.ClearComments
.AddComment "This is a very big, very long comment to test sizing."
Set comA1 = .Comment
End With

With comA1.Shape
.Height = 100
.Width = 50
End With
End Sub

--
Regards,
Bill Renaud