Trade Officer
The minimum recommended radius is 3ft. However it depends on what locos and stock you intend running. An 0-4-0 for instance could tolerate a smaller radius than a 6 or 8 coupled loco or a 6 wheeled rigid van. A 10 coupled loco may not get around a 3ft radius smoothly.
I have seen dockyard layouts with 2ft radii but they only have very short wheelbase stock. So it is somewhat dependent on the type of layout – very much like the prototype really
Gauge widening will help getting the larger wheelbase stock round the curve if space is a problem as will a little bit extra side play
john