Boundary conditions

Guys, i have to find the electric field in a microscopic piece of a irregular conductive surface neighborhood. The potential of the surface is V with regard to the infinity. How can I represent the infinity? I’ve created the 2D surface and put 4 extra points, closing a volume. Thus, I have something that seems like a cube (conductive surface = ‘‘base’’, 4 side surfaces and the top). For the conductive surface it is obvious to use the Dirichilet BC. For the sides maybe Neumann BC. For the top I really don’t know. Can someone help me?