How do we accurately communicate the true sizes of a building's parts?

 Architectural drawings are made to scale, meaning that a given length on a drawing corresponds to a specific length in reality. For example, a floor plan could be drawn at a scale of 1/4" = 1'-0", meaning that each quarter inch on the drawing represents one foot of the floor plan's actual size. So, at this scale, a doorway that is 3'-0" wide would be 3/4" wide in the drawing.

A drawing's scale often corresponds with what type of information the drawing is intended to communicate. For example, a site plan, which typically shows not only a building but a large portion of its surroundings, is usually drawn at a much smaller scale (i.e. 1" = 20'-0") than something like the interior plan of a single bedroom (i.e. 1/4" = 1'-0" or 1/2" = 1'-0"), which is in turn smaller than the scale used to depict the joint between a window and its frame (i.e. 1/4" = 1"). In other words, the scale of a drawing is usually related to the level of detail that you want to show.

https://the-bac.libguides.com/Scale_Drawing#:~:text=Architectural%20drawings%20are%20made%20to,the%20floor%20plan's%20actual%20size.