A reasonable minimum
curve radius in HO is 18", although 15" is usable with the right
equipment. A half circle to reverse the direction
of a train is therefore 36" or 3' across. A space wide enough to
accommodate this must be at least 3.5 feet. A better minimum
radius is 24" which means a 4 foot wide area isn't big enough. A
24" radius is marginal for full length (85') passenger cars.
Multiply by 2 for O scale or divide by 2 for N scale and you begin to
see the attraction of the smaller scales.