If you are asking about wheel base, this could be anywhere between 30" to 42". The rest depends entirely on the original designer.
Email me and I will send you the rolling stock for the Chicago Tunnel Company which was 2 ft gauge. You could use those bogies. If it helps you can have the GMax files. After you see the stock let me know about the files.
As far as locos are concerned, I usually take the gauge and subtract 3 inches. That is the distance between the back of the wheel flanges, in this case 1ft 9in