If you want to store the beam, you need a ring. And the size of the ring is given by the minimum bending radius, which is given by maximum field strength you can achieve, and energy loss from bremsstrahlung in the bends, which you have to put back.

So space doesn't really help: You'll still need a circle of magnets. Approximating the circle with a polygon means you split up a smaller circle with straight segments. But these straight segments are not really useful.