If you go with an 18" circle, you will need twelve 18" radius curves.
If you go with a 22" circle, you will need sixteen 22" radius curves.
If you have a small enough loco and cars, you could try 15" radius. A 15" radius circle would be 32" wide and you would have 8" on each side. For the circle you would need twelve 15" radius curves.
You could add a 9" straight and a 3" straight for 12 inches.
That would leave you with 2" on each side. 32+12=44" wide
If you do an oval with 15", you will also need 2 pieces of both 9" and 3" straight or you can can use either the 9" or 3".
For a square you need four 9" and 3".
For the 18" oval or square it is the same number of pieces with 4.5" and 3" instead of 9" and 3".
If you go with a 22" circle, you will need sixteen 22" radius curves.
If you have a small enough loco and cars, you could try 15" radius. A 15" radius circle would be 32" wide and you would have 8" on each side. For the circle you would need twelve 15" radius curves.
You could add a 9" straight and a 3" straight for 12 inches.
That would leave you with 2" on each side. 32+12=44" wide
If you do an oval with 15", you will also need 2 pieces of both 9" and 3" straight or you can can use either the 9" or 3".
For a square you need four 9" and 3".
For the 18" oval or square it is the same number of pieces with 4.5" and 3" instead of 9" and 3".