I have to wonder why (in 2024) you are using formfields instead of content controls in your form. Regardless:

Sub FFOnExit()
Dim oFld As Word.FormField
  If Selection.FormFields.Count = 1 Then
    Set oFld = ActiveDocument.FormFields(Selection.FormFields(1).Name)
  ElseIf Selection.FormFields.Count = 0 And Selection.Bookmarks.Count > 0 Then
    Set oFld = ActiveDocument.FormFields(Selection.Bookmarks(Selection.Bookmarks.Count).Name)
  End If
  With oFld
    Select Case Len(.Result)
       Case Is < 74
         .Range.Font.Size = 14
       Case 74 To 80
         .Range.Font.Size = 13
       Case 80 To 90
         .Range.Font.Size = 12
       Case Is > 90
         .Range.Font.Size = 11
    End Select
  End With
lbl_Exit:
  Exit Sub
End Sub