This source discusses Ampere's Force Law. The force between two parallel conductors is equal to two times a constant, times the current in wire A, times the current in wire B, divided by the spacing between wires. The constant is known as the magnetic force constant and is 1 x 10E-7 newtons in the SI system.
For parallel conductors one meter in length, one meter apart and carrying one ampere, the force between them is exactly 2 x 10E-7 newtons.
The force in a rail gun will be directly proportional to its current and inversely proportional to the spacing between its rails.
But what length should you assume when calculating a rail gun? The length of the conductor should be considered to be the width of the projectile. This is equal to the distance between rails. As the distance between rails decreases, the unit forces increase, but the projectile gets smaller. Those two considerations fall out and it makes no difference what the rail spacing is. In other words, make the rail spacing as small as possible in order to accommodate your projectile.