Hi Joe,
Are you talking about just surfacing a regular rectangular area flat? In that case you can just create a project and a Parallel Carving operation with the cutting direction set to Mixed (or if you specifically want Conventional/Climb you can do that too) and just let it generate a toolpath that fills the entire canvas. Then it's just a matter of designating the dimensions of the canvas. You also can set the Canvas Resolution pretty low too as there's no height information that it needs to worry about contouring.
- Charlie