How do you change Point Size in Direct3d 11 shader?

I'm rendering a point cloud using MeshTopology.Points. I wrote a simple vertex shader that can change the point size during play and that worked fine until Direct3D 11. I have to use D3D11 because I'm using the Rift DK2. Does anyone know the right shader code to make this work in Unity 4.5.5 and D3D11?