Depend on what you like to run as to how big the curves need to be. Tubular track curves start at 27 inches(027) and go up to 72 inches(072) in Lionel style curves. Gargraves makes curves that start at 32 inches and go up to 120 inches, maybe higher.
For 027 curves it makes a 27 inch circle, 072 makes about a 72 inch circle(plus pr minus a half inch due to track play). A 30 inch square would be the smallest for 027 curves and for 031 a 3 foot square would be about the smallest.
Lee Fritz
