Give this a try

Dim Cell As Range

For Each Cell In Worksheets("Sheet1").Range("A8", "A:A")
    With Cell
        If Trim(.Value) <> "" Then
            Select Case Len(.Value)
                Case Is = 16
                    .Value = "C" & .Value
                    MsgBox ("test 16")
                Case Is <> 9
                    .ClearContents
                    MsgBox ("test <> 9 but not 16")
            End Select
        End If
    End With
Next Cell