The model number is a model number, it is not a length. If a manufacturer applies any meaning to the number, it more often helps to rank individual models by where the manufacturer wants the price point to be. Even from the same manufacturer something with a 29 in the number could be three feet longer than something with 30 in the number.
The manufacturer will have length specifications for each model. These will be tongue to rear bumper (if it has a bumper). It will not necessarily include length added by accessories like rear-mounted spare or a ladder, so be aware of that if shopping to fit a garage or driveway space.
