ClearCube v0.6

args = {...}
width = args[1];
length = args[2];
height = args[3];

turtle.turnRight();
for i=0, width/2-1, 1 do
    --Assuming that we were put in the centre of one side of the bottom of the cube
    -- move to the side of the cube to start.
    turtle.dig();
        while not turtle.forward() do
            turtle.dig()
        end
    turtle.turnLeft();
end


function mineRow(num)
    for i=0, num, 1 do
    --Dig infront of you.
        while turtle.detect() do
            turtle.dig()
        end
    --dig infront of you if you can't move forward.
        while not turtle.forward() do
            turtle.dig()
        end
    end
end

function digPlane(x, y)
    for i=0, y, 2 do
        --Mine 1 row
        mineRow(x);
        turtle.turnRight();
        --Mine column a little
        mineRow(1);
        turtle.turnRight();
        --Mine 1 row
        mineRow(x);
        turtle.turnLeft();
        --Exit if we've finished this area.
        if i+2 >= y then
            break
        end
        --Else turn around and get ready for the next one.
        mineRow(1);
        turtle.turnLeft();
    end
    --Turn around and get ready for the next plane.
    turtle.turnLeft();
end

function digCube(x, y, z)
    for i=0, z, 1 do
        digPlane(x, y);
        --Dig above you
            while turtle.detectUp() do
                turtle.digUp()
            end
        --Move up
            while not turtle.up() do
                turtle.digUp()
            end
    end
end

digCube(length, width, height);