Andrew Ullyott
As with so many questions in life; it depends.
4ft is often quoted as the desirable minimum but you should be able to get down to 2ft 6in in EM without too much bother.
It depends what you’re running, how fast you’re running it and how much side play is built into the axles.
8 or 10 coupled locos are going to be a an issue but 0-4-0’s will go down to less than that.