I am sorry for stupid answer, but what force is supposed to stop the plasma? Gravity, interaction with external neutrals, etc? Unbound plasma in vacuum will expand into infinity, that is well known classical problem of ambipolar diffusion. If you have timescale, you can estimate how far the diffusion will go, at least.

in DC plasma you can control the width of a plasma by forcing the plasma to be controlled with metallic walls. the hole in the plasma container can force the plasma to be less than. An example is published in spectrochmica Acta B 1983. "Spectrographic determination of rare earth elements using a wallstabilized plasma arc" link  : http://dx.doi.org/10.1016/0584-8547(83)80023-8

For stopping the plasma expansion, there must be some force. In vacuum only the collision of plasma with neutral can stop the free expansion of plasma. The presence of some buffer gas can confine the plasma. In this case stopping radius will be decided by the pressure of the neutral buffer gas. In the absence of magnetic field and buffer gas plasma can diffuse even up to infinity. So in the absence of magnetic field only buffer gas can stop the plasma.