Generate a circle mesh?

First, usually a circle mesh would be build in a 3D modeling program like Blender and then imported into Unity. You would then scale and position the mesh. If you really want to build on at runtime, you can generate an array of points around the circumference of the circle and use the Triangular script from the Unity Wiki to change the points into a mesh. Note rather than use an arbitrary center, use Vector3.zero for the center, then once the mesh is attached to a game object, you can move the game object to the center position.