Create plane in cam's view frustum x units away

I'm stumped on creating a plane that fits a camera's view frustum. I used GeometryUtils.CalculateFrustumPlanes but there is no way to scale the plane to the correct size. The game will be played on screens with different aspect ratios so I know that needs to be taken into account. The camera always points down. I'm sure there is a way using math but I cant find anything after searching.