Speculations on the source of the 9mm gauge for 2mm modelling, with some observations on the Theory of Scale (with apologies to C. Dickens and S. Pickwick). 

The gauge of the track is the distance between the rails which, in the case of standard gauge track, as used in Britain and large parts of the world, is 4'-8½" (1435.1mm) The gauge of the track for N Gauge is 9mm

 The scale of a model can be stated two ways, either as a simple ratio e.g. 1 to 50, normally written 1:50 or, as is common in model railways, so many millimetres to one foot. For N Gauge this is stated as 2mm per foot but it is also stated as 1:148 (in Britain)

 If we do a little arithmetic (not maths and certainly not math!) we start to see a few issues 2mm per foot is 1:152 based on the following. 25.4 mm in one inch so a foot is 12x25.4=304.8mm. Normal rounding would give us 305mm and a scale of 1:152.5 but this is not very clean and rounding down gives a more usable scale

 At 1:152 the gauge should be 1435.1/152 = 9.44mm or to put it another way 9mm at 1:152 = 9x152=1368mm or 4'-5.86" call it 4'-6"

 At 1:148 the gauge should be 1435.1/148 = 9.70mm or to put it another way 9mm at 1:1148 = 9x148=1332mm or 4'-4.44" call it 4'-4"

 At 1:160 the gauge should be 1435.1/160 = 8.97mm or to put it another way 9mm at 1:160 = 9x160=1440mm or 4'-8.69" call it 4'-8½"

 And there you have it. The original scale for N Gauge was intended to be 1:160 but the locomotive body size which this produced would not accommodate the motors available at the time (1960ish) and so 1:148 was adopted because it was!

 So now the question has to be, when modelling a building which is, say, 100 feet long should you make it 200mm long or 206mm long. My vote goes for 200mm