This webpage has some good info for measurements...
Also, 1:400 means that 400 inches in real life = 1 inch in your model, so a runway is usually 150ft wide 150 x 12 (12 inches/ft) = 1800 inches wide (real life) / 400 = 4.5 inches wide in your model.
so 12,000ft runway = 144,000 inches / 400 = 360 inches / 12 = 30ft long in your model.
The website also gives references to the FAA website for Advisory Circulars (ACs), the ACs governing airport construction in the real world are the AC 150 series which give design guidelines for terminals, gates, runways, runway markings, runway lighting, etc...
Basically, designers used three different size "gates" for aircraft, based on the airplane's wingspan. The largest size gates are designed for A380/747 size aircraft, down to the 737/CRJ-sized aircraft for the smallest gates..
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.