Answer:
To construct a model to scale, you need to know the measurements of the full size object , and then once you have decided what scale you wish to model at, you create all items for the model to the same relative proportions of the original.
For example a 1:100 scale model has all the linear measurements at 1/100th of the size of the original. A 100m long wall would be modeled at 1m long. A 1m wide door would be modeled at 0.01m wide.