Hesperian,

Well, I would have thought that changing the Field Size property of a
field, which in practice does indeed need to be restricted, would not be
violating the "make sure that the look and feel of the application does
not change much from what it already is" rule.

Anyway, untested "air code"...

 Public Function TextTooLong(InputText As String) As Boolean
    Dim CRs As Integer
    Dim TextLength As Integer
    Dim TestText As String
    Dim CRPosition As Integer
    TestText = InputText
    TextLength = Len(InputText)
    CRPosition = InStr(TestText, Chr(13) & Chr(10))
    Do Until CRPosition = 0
        CRs = CRs + 1
        TestText = Mid(TestText, CRPosition + 1)
        CRPosition = InStr(TestText, Chr(13) & Chr(10))
    Loop
    If TextLength > 120 _
      Or CRs = 1 And TextLength > 100 _
      Or CRs = 2 And TextLength > 70
      Or CRs > 2 Then
          TextTooLong = True
    End If
 End Function

And then, on the AfterUpdate event of the applicable textbox on your form...
 If TextTooLong(Me.NameOfTextbox) Then
   MsgBox "Hey, this is too long to fit on the report!"
 End If

Of course, the "rules" about length of text and number of carriage
returns allowable will necessarily be arbitrary, and will sometimes flag
as too long stuff that will in fact easily fit, and may at other times
allow an entry that is in fact too long, so this will not be a precise
science.