Trade Officer
Hi Garry
Although there is no hard a fast rule, I would normally work to 3ft (in old money) radius. you can probably get away with less depending on how much side play you have on the wheels and whether you have any long wheel base rigid stock.
Design your layout with the maximum radius you can fit and then build a shortish length of track with whatever method you plan to use and run your engines / stock over it.
With scratch built track gauge widening can help
John